diff --git a/dashboard/e2e/tree-compare.spec.ts b/dashboard/e2e/tree-compare.spec.ts
index 3d2fddcb2..19a4fd1d6 100644
--- a/dashboard/e2e/tree-compare.spec.ts
+++ b/dashboard/e2e/tree-compare.spec.ts
@@ -138,7 +138,7 @@ test('loads comparison data and opens a side-by-side details drawer', async ({
   );
 
   await page.goto(
-    `/tree/linux/master/compare?hashA=${HASH_A}&hashB=${HASH_B}&origin=maestro`,
+    `/tree/linux/master/compare?range=${HASH_A}..${HASH_B}&origin=maestro`,
   );
 
   await expect(page.getByText('Tree summary')).toBeVisible();
@@ -304,7 +304,7 @@ test('drawer next stays on searched rows', async ({ page }) => {
   );
 
   await page.goto(
-    `/tree/linux/master/compare?hashA=${HASH_A}&hashB=${HASH_B}&origin=maestro`,
+    `/tree/linux/master/compare?range=${HASH_A}..${HASH_B}&origin=maestro`,
   );
 
   await page.getByPlaceholder('Search').fill('keep-');
diff --git a/dashboard/src/locales/messages/index.ts b/dashboard/src/locales/messages/index.ts
index 5d2dada70..7804ebcfd 100644
--- a/dashboard/src/locales/messages/index.ts
+++ b/dashboard/src/locales/messages/index.ts
@@ -417,6 +417,7 @@ export const messages = {
     'tree.path': 'Trees',
     'tree.searchPlaceholder': 'Search by tree, branch or tag with a regex',
     'treeCompare.backToDetails': 'Back to tree details',
+    'treeCompare.base': 'Base',
     'treeCompare.breadcrumb': 'Compare',
     'treeCompare.breakdownTitle': 'Changed results',
     'treeCompare.change.appeared': 'Appeared',
@@ -431,11 +432,12 @@ export const messages = {
     'treeCompare.change.stillFailing': 'Still failing',
     'treeCompare.change.unchanged': 'Unchanged',
     'treeCompare.changed': 'Changed',
+    'treeCompare.compare': 'Compare',
     'treeCompare.description':
-      'Compare pass/fail counts between two revisions on the same tree and branch.',
-    'treeCompare.detail.missingSide': 'No result on this side',
+      'Compare pass/fail counts of a revision against a base revision on the same tree and branch.',
+    'treeCompare.detail.missingSide': 'No result on this revision',
     'treeCompare.drilldownHint':
-      'Individual builds, boots, and tests that changed between Side A and Side B.',
+      'Individual builds, boots, and tests that changed from the base revision to the compared revision.',
     'treeCompare.failures.change': 'Change',
     'treeCompare.failures.configArch': 'Config / Arch',
     'treeCompare.failures.pathArch': 'Path / Arch',
@@ -454,8 +456,6 @@ export const messages = {
       'Select two different revisions to compare. This tree needs at least two commits with results.',
     'treeCompare.openCompare': 'Compare revisions',
     'treeCompare.selectRevision': 'Select a revision',
-    'treeCompare.sideA': 'Side A',
-    'treeCompare.sideB': 'Side B',
     'treeCompare.statusPairFilter.active': 'Active status pair filters',
     'treeCompare.statusPairFilter.add': 'Add',
     'treeCompare.statusPairFilter.from': 'From',
@@ -463,13 +463,13 @@ export const messages = {
       'PASS — completed successfully{br}' +
       'FAIL — completed with a failure{br}' +
       'INCONCLUSIVE — no definitive pass or fail result{br}' +
-      '— — absent on this side',
+      '— — absent on this revision',
     'treeCompare.statusPairFilter.remove': 'Remove {from} to {to} filter',
     'treeCompare.statusPairFilter.select': 'Select status',
     'treeCompare.statusPairFilter.to': 'To',
     'treeCompare.suggestion.branchHead': 'Branch head',
     'treeCompare.suggestion.previous': 'Previous commit',
-    'treeCompare.suggestion.swap': 'Swap sides',
+    'treeCompare.suggestion.swap': 'Swap base and compare',
     'treeCompare.suggestions': 'Suggestions',
     'treeCompare.summaryTitle': 'Tree summary',
     'treeDetails.bootsHistory': 'Boots History',
diff --git a/dashboard/src/pages/TreeCompare/TreeCompareLink.tsx b/dashboard/src/pages/TreeCompare/TreeCompareLink.tsx
index 48806f30b..14931daf0 100644
--- a/dashboard/src/pages/TreeCompare/TreeCompareLink.tsx
+++ b/dashboard/src/pages/TreeCompare/TreeCompareLink.tsx
@@ -6,6 +6,8 @@ import { FormattedMessage } from 'react-intl';
 
 import { Button } from '@/components/ui/button';
 
+import { formatCompareRange } from '@/utils/treeCompareDiff';
+
 interface TreeCompareLinkProps {
   treeName: string;
   branch: string;
@@ -24,7 +26,10 @@ export function TreeCompareLink({
       <Link
         to="/tree/$treeName/$branch/compare"
         params={{ treeName, branch }}
-        search={{ hashA: hash, hashB: '', origin }}
+        search={{
+          range: formatCompareRange({ base: '', compare: hash }),
+          origin,
+        }}
         state={s => s}
       >
         <GitCompareArrows className="mr-2 h-4 w-4" />
diff --git a/dashboard/src/pages/TreeCompare/TreeComparePage.tsx b/dashboard/src/pages/TreeCompare/TreeComparePage.tsx
index 078b279c5..aea738ac8 100644
--- a/dashboard/src/pages/TreeCompare/TreeComparePage.tsx
+++ b/dashboard/src/pages/TreeCompare/TreeComparePage.tsx
@@ -41,10 +41,14 @@ import {
 } from '@/types/tree/TreeCompare';
 import type { PossibleTabs } from '@/types/tree/TreeDetails';
 import {
+  type CompareRange,
   applyStatusPairFilter,
+  formatCompareRange,
   mapBootOrTestDiffRows,
   mapBuildDiffRows,
+  parseCompareRange,
   readStoredStatusPairs,
+  resolveCompareRange,
   resolveStatusPairs,
   serializeStatusPairs,
   writeStoredStatusPairs,
@@ -57,16 +61,20 @@ import {
 } from './components/CompareFailuresTables';
 import { CompareTestsGroupedTable } from './components/CompareTestsGroupedTable';
 import { CompareSummary } from './components/CompareSummary';
-import { RevisionSelectorBar } from './components/RevisionSelector';
+import {
+  RevisionSelectorBar,
+  type RevisionSide,
+} from './components/RevisionSelector';
 
 const SHORT_HASH_LENGTH = 7;
 
 const TreeComparePage = (): JSX.Element => {
   const { formatMessage } = useIntl();
   const { treeName, branch } = useParams({ from: compareRouteName });
-  const { hashA, hashB, origin, currentPageTab, statusPair } = useSearch({
+  const { range, origin, currentPageTab, statusPair } = useSearch({
     from: compareRouteName,
   });
+  const { base: baseHash, compare: compareHash } = parseCompareRange(range);
   const navigate = useNavigate({ from: compareNavigateFrom });
 
   const commitsQuery = useCommits({
@@ -88,19 +96,19 @@ const TreeComparePage = (): JSX.Element => {
     [commitsQuery.data],
   );
 
-  const resolvedHashA = hashA || revisions[0]?.hash || '';
-  const resolvedHashB =
-    hashB ||
-    revisions.find(revision => revision.hash !== resolvedHashA)?.hash ||
-    '';
+  const { base: resolvedBase, compare: resolvedCompare } = useMemo(
+    () =>
+      resolveCompareRange(revisions, { base: baseHash, compare: compareHash }),
+    [revisions, baseHash, compareHash],
+  );
 
-  const canCompare = Boolean(resolvedHashA && resolvedHashB);
+  const canCompare = Boolean(resolvedBase && resolvedCompare);
 
   const compareParams = {
     treeName,
     branch,
-    hashA: resolvedHashA,
-    hashB: resolvedHashB,
+    hashA: resolvedBase,
+    hashB: resolvedCompare,
     origin,
   };
 
@@ -111,16 +119,17 @@ const TreeComparePage = (): JSX.Element => {
 
   const updateSearch = useCallback(
     (updates: {
-      hashA?: string;
-      hashB?: string;
+      range?: Partial<CompareRange>;
       currentPageTab?: PossibleTabs;
       statusPair?: string[];
     }) => {
       navigate({
         search: previous => ({
           ...previous,
-          hashA: updates.hashA ?? previous.hashA,
-          hashB: updates.hashB ?? previous.hashB,
+          range: formatCompareRange({
+            ...parseCompareRange(previous.range),
+            ...updates.range,
+          }),
           currentPageTab:
             updates.currentPageTab ??
             previous.currentPageTab ??
@@ -135,18 +144,18 @@ const TreeComparePage = (): JSX.Element => {
   );
 
   useEffect(() => {
-    if ((!hashA || !hashB) && resolvedHashA && resolvedHashB) {
-      updateSearch({ hashA: resolvedHashA, hashB: resolvedHashB });
+    if ((!baseHash || !compareHash) && resolvedBase && resolvedCompare) {
+      updateSearch({ range: { base: resolvedBase, compare: resolvedCompare } });
     }
-  }, [hashA, hashB, resolvedHashA, resolvedHashB, updateSearch]);
+  }, [baseHash, compareHash, resolvedBase, resolvedCompare, updateSearch]);
 
   const handleSwap = useCallback(() => {
-    updateSearch({ hashA: resolvedHashB, hashB: resolvedHashA });
-  }, [resolvedHashA, resolvedHashB, updateSearch]);
+    updateSearch({ range: { base: resolvedCompare, compare: resolvedBase } });
+  }, [resolvedBase, resolvedCompare, updateSearch]);
 
   const handleSideAction = useCallback(
-    (side: 'A' | 'B', action: 'previous' | 'branchHead') => {
-      const currentHash = side === 'A' ? resolvedHashA : resolvedHashB;
+    (side: RevisionSide, action: 'previous' | 'branchHead') => {
+      const currentHash = side === 'base' ? resolvedBase : resolvedCompare;
       const currentIndex = revisions.findIndex(r => r.hash === currentHash);
 
       if (action === 'previous') {
@@ -155,22 +164,14 @@ const TreeComparePage = (): JSX.Element => {
           Math.max(currentIndex, 0) + 1,
         );
         const nextHash = revisions[previousIndex]?.hash ?? currentHash;
-        if (side === 'A') {
-          updateSearch({ hashA: nextHash });
-        } else {
-          updateSearch({ hashB: nextHash });
-        }
+        updateSearch({ range: { [side]: nextHash } });
         return;
       }
 
       const headHash = revisions[0]?.hash ?? currentHash;
-      if (side === 'A') {
-        updateSearch({ hashA: headHash });
-      } else {
-        updateSearch({ hashB: headHash });
-      }
+      updateSearch({ range: { [side]: headHash } });
     },
-    [revisions, resolvedHashA, resolvedHashB, updateSearch],
+    [revisions, resolvedBase, resolvedCompare, updateSearch],
   );
 
   const buildRows = useMemo(
@@ -317,7 +318,7 @@ const TreeComparePage = (): JSX.Element => {
                 params={{
                   treeName,
                   branch,
-                  hash: resolvedHashA,
+                  hash: resolvedCompare,
                 }}
                 state={s => s}
               >
@@ -345,8 +346,8 @@ const TreeComparePage = (): JSX.Element => {
               origin,
               gitUrl: compareQuery.data?.gitUrl,
               branch,
-              hashA: resolvedHashA || undefined,
-              hashB: resolvedHashB || undefined,
+              hashA: resolvedBase || undefined,
+              hashB: resolvedCompare || undefined,
               omittedFilters:
                 statusPairs.length > 0 ? ['status-pair filter'] : [],
             })}
@@ -362,7 +363,7 @@ const TreeComparePage = (): JSX.Element => {
             </div>
             <Link
               to="/tree/$treeName/$branch/$hash"
-              params={{ treeName, branch, hash: resolvedHashA }}
+              params={{ treeName, branch, hash: resolvedCompare }}
               className="text-blue text-sm font-medium hover:underline"
               state={s => s}
             >
@@ -375,11 +376,13 @@ const TreeComparePage = (): JSX.Element => {
             error={commitsQuery.error}
           >
             <RevisionSelectorBar
-              hashA={resolvedHashA}
-              hashB={resolvedHashB}
+              baseHash={resolvedBase}
+              compareHash={resolvedCompare}
               revisions={revisions}
-              onHashAChange={value => updateSearch({ hashA: value })}
-              onHashBChange={value => updateSearch({ hashB: value })}
+              onBaseChange={value => updateSearch({ range: { base: value } })}
+              onCompareChange={value =>
+                updateSearch({ range: { compare: value } })
+              }
               onSideAction={handleSideAction}
               onSwap={handleSwap}
             />
diff --git a/dashboard/src/pages/TreeCompare/components/CompareDetailSheet.tsx b/dashboard/src/pages/TreeCompare/components/CompareDetailSheet.tsx
index fba25970d..013309b6b 100644
--- a/dashboard/src/pages/TreeCompare/components/CompareDetailSheet.tsx
+++ b/dashboard/src/pages/TreeCompare/components/CompareDetailSheet.tsx
@@ -64,7 +64,7 @@ function SideColumn({
   logData,
   isLoading,
 }: {
-  labelId: 'treeCompare.sideA' | 'treeCompare.sideB';
+  labelId: 'treeCompare.base' | 'treeCompare.compare';
   status: CompareItemStatus;
   id: string | null;
   logType: LogType;
@@ -154,7 +154,7 @@ export function CompareDetailSheet({
             </div>
             <div className="grid gap-6 lg:grid-cols-2">
               <SideColumn
-                labelId="treeCompare.sideA"
+                labelId="treeCompare.base"
                 status={item.sideA}
                 id={item.idA}
                 logType={logType}
@@ -162,7 +162,7 @@ export function CompareDetailSheet({
                 isLoading={logA.isLoading}
               />
               <SideColumn
-                labelId="treeCompare.sideB"
+                labelId="treeCompare.compare"
                 status={item.sideB}
                 id={item.idB}
                 logType={logType}
diff --git a/dashboard/src/pages/TreeCompare/components/CompareFailuresTables.tsx b/dashboard/src/pages/TreeCompare/components/CompareFailuresTables.tsx
index 2cc63f4a4..6940ace26 100644
--- a/dashboard/src/pages/TreeCompare/components/CompareFailuresTables.tsx
+++ b/dashboard/src/pages/TreeCompare/components/CompareFailuresTables.tsx
@@ -268,7 +268,7 @@ export function CompareBuildsFailuresTable({
             />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideA"
+              intlKey="treeCompare.base"
               sortKey="sideA"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}
@@ -276,7 +276,7 @@ export function CompareBuildsFailuresTable({
             <TableHead className="bg-light-gray w-8" />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideB"
+              intlKey="treeCompare.compare"
               sortKey="sideB"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}
@@ -408,7 +408,7 @@ export function CompareBootsFailuresTable({
             />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideA"
+              intlKey="treeCompare.base"
               sortKey="sideA"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}
@@ -416,7 +416,7 @@ export function CompareBootsFailuresTable({
             <TableHead className="bg-light-gray w-8" />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideB"
+              intlKey="treeCompare.compare"
               sortKey="sideB"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}
diff --git a/dashboard/src/pages/TreeCompare/components/CompareTestsGroupedTable.tsx b/dashboard/src/pages/TreeCompare/components/CompareTestsGroupedTable.tsx
index 7de4d463c..79956cef3 100644
--- a/dashboard/src/pages/TreeCompare/components/CompareTestsGroupedTable.tsx
+++ b/dashboard/src/pages/TreeCompare/components/CompareTestsGroupedTable.tsx
@@ -232,7 +232,7 @@ function createCompareSideColumns(
         sideStatus(row, 'sideA'),
       header: ({ column }): JSX.Element => (
         <div className="flex justify-center">
-          <SortableTableHeader column={column} intlKey="treeCompare.sideA" />
+          <SortableTableHeader column={column} intlKey="treeCompare.base" />
         </div>
       ),
       cell: ({ row }): JSX.Element | null => {
@@ -273,7 +273,7 @@ function createCompareSideColumns(
         sideStatus(row, 'sideB'),
       header: ({ column }): JSX.Element => (
         <div className="flex justify-center">
-          <SortableTableHeader column={column} intlKey="treeCompare.sideB" />
+          <SortableTableHeader column={column} intlKey="treeCompare.compare" />
         </div>
       ),
       cell: ({ row }): JSX.Element | null => {
diff --git a/dashboard/src/pages/TreeCompare/components/RevisionSelector.tsx b/dashboard/src/pages/TreeCompare/components/RevisionSelector.tsx
index ca68b07fb..93ad42641 100644
--- a/dashboard/src/pages/TreeCompare/components/RevisionSelector.tsx
+++ b/dashboard/src/pages/TreeCompare/components/RevisionSelector.tsx
@@ -16,7 +16,7 @@ import type { CompareRevision } from '@/types/tree/TreeCompare';
 
 import { cn } from '@/lib/utils';
 
-type RevisionSide = 'A' | 'B';
+export type RevisionSide = 'base' | 'compare';
 
 function TagChips({ tags }: { tags: string[] }): JSX.Element | null {
   if (tags.length === 0) {
@@ -58,24 +58,14 @@ function RevisionCard({
     <div
       className={cn(
         'flex flex-1 flex-col gap-3 rounded-lg border bg-white p-4',
-        side === 'A' ? 'border-blue/40' : 'border-dim-gray/30',
+        side === 'base' ? 'border-blue/40' : 'border-dim-gray/30',
       )}
     >
-      <div className="flex items-center gap-2">
-        <span
-          className={cn(
-            'inline-flex h-7 w-7 items-center justify-center rounded-full text-sm font-bold text-white',
-            side === 'A' ? 'bg-blue' : 'bg-dim-gray',
-          )}
-        >
-          {side}
-        </span>
-        <span className="text-dim-black text-sm font-semibold">
-          <FormattedMessage
-            id={side === 'A' ? 'treeCompare.sideA' : 'treeCompare.sideB'}
-          />
-        </span>
-      </div>
+      <span className="text-dim-black text-sm font-semibold">
+        <FormattedMessage
+          id={side === 'base' ? 'treeCompare.base' : 'treeCompare.compare'}
+        />
+      </span>
 
       <Select value={selectedHash} onValueChange={onSelect}>
         <SelectTrigger className="w-full">
@@ -145,21 +135,21 @@ function RevisionCard({
 }
 
 interface RevisionSelectorBarProps {
-  hashA: string;
-  hashB: string;
+  baseHash: string;
+  compareHash: string;
   revisions: CompareRevision[];
-  onHashAChange: (hash: string) => void;
-  onHashBChange: (hash: string) => void;
+  onBaseChange: (hash: string) => void;
+  onCompareChange: (hash: string) => void;
   onSideAction: (side: RevisionSide, action: 'previous' | 'branchHead') => void;
   onSwap: () => void;
 }
 
 export function RevisionSelectorBar({
-  hashA,
-  hashB,
+  baseHash,
+  compareHash,
   revisions,
-  onHashAChange,
-  onHashBChange,
+  onBaseChange,
+  onCompareChange,
   onSideAction,
   onSwap,
 }: RevisionSelectorBarProps): JSX.Element {
@@ -167,12 +157,12 @@ export function RevisionSelectorBar({
     <div className="flex flex-col gap-4">
       <div className="flex flex-col items-stretch gap-4 lg:flex-row lg:items-center">
         <RevisionCard
-          side="A"
-          selectedHash={hashA}
+          side="base"
+          selectedHash={baseHash}
           revisions={revisions}
-          onSelect={onHashAChange}
-          onPrevious={() => onSideAction('A', 'previous')}
-          onBranchHead={() => onSideAction('A', 'branchHead')}
+          onSelect={onBaseChange}
+          onPrevious={() => onSideAction('base', 'previous')}
+          onBranchHead={() => onSideAction('base', 'branchHead')}
         />
 
         <div className="flex shrink-0 items-center justify-center">
@@ -180,7 +170,7 @@ export function RevisionSelectorBar({
             type="button"
             onClick={onSwap}
             className="bg-medium-gray flex h-10 w-10 items-center justify-center rounded-full"
-            aria-label="Swap sides"
+            aria-label="Swap base and compare"
           >
             <ArrowLeftRight className="text-dim-gray h-5 w-5" />
             <span className="sr-only">
@@ -190,12 +180,12 @@ export function RevisionSelectorBar({
         </div>
 
         <RevisionCard
-          side="B"
-          selectedHash={hashB}
+          side="compare"
+          selectedHash={compareHash}
           revisions={revisions}
-          onSelect={onHashBChange}
-          onPrevious={() => onSideAction('B', 'previous')}
-          onBranchHead={() => onSideAction('B', 'branchHead')}
+          onSelect={onCompareChange}
+          onPrevious={() => onSideAction('compare', 'previous')}
+          onBranchHead={() => onSideAction('compare', 'branchHead')}
         />
       </div>
     </div>
diff --git a/dashboard/src/types/tree/TreeCompare.ts b/dashboard/src/types/tree/TreeCompare.ts
index c77b7005c..19f506496 100644
--- a/dashboard/src/types/tree/TreeCompare.ts
+++ b/dashboard/src/types/tree/TreeCompare.ts
@@ -138,15 +138,14 @@ export type CompareFailureRow =
   | CompareTestFailureRow;
 
 export const compareDefaultValues = {
-  hashA: '',
-  hashB: '',
+  range: '',
   origin: 'maestro',
   currentPageTab: 'global.builds' as const,
 };
 
 export const compareSearchSchema = z.object({
-  hashA: z.string().catch(''),
-  hashB: z.string().catch(''),
+  /** `<base>..<compare>` commit hashes; see `parseCompareRange`. */
+  range: z.string().catch(''),
   origin: z
     .string()
     .default(compareDefaultValues.origin)
diff --git a/dashboard/src/utils/treeCompareDiff.test.ts b/dashboard/src/utils/treeCompareDiff.test.ts
index 46d388d83..4e6e53a9e 100644
--- a/dashboard/src/utils/treeCompareDiff.test.ts
+++ b/dashboard/src/utils/treeCompareDiff.test.ts
@@ -1,12 +1,16 @@
 import { describe, expect, it } from 'vitest';
 
 import {
+  type CompareRange,
   deriveCompareChange,
   applyStatusPairFilter,
+  formatCompareRange,
   mapBootOrTestDiffRows,
   mapBuildDiffRows,
   compareRowNav,
+  parseCompareRange,
   parseStatusPairs,
+  resolveCompareRange,
   resolveStatusPairs,
   serializeStatusPairs,
   toggleChangeTypePairs,
@@ -44,6 +48,62 @@ describe('deriveCompareChange', () => {
   });
 });
 
+describe('compare range URL param', () => {
+  it('round-trips <base>..<compare>, with either side empty', () => {
+    expect(parseCompareRange('old..new')).toEqual({
+      base: 'old',
+      compare: 'new',
+    });
+    expect(parseCompareRange('..new')).toEqual({ base: '', compare: 'new' });
+    expect(parseCompareRange('old..')).toEqual({ base: 'old', compare: '' });
+    expect(parseCompareRange('')).toEqual({ base: '', compare: '' });
+    expect(formatCompareRange({ base: 'old', compare: 'new' })).toBe(
+      'old..new',
+    );
+    expect(formatCompareRange({ base: '', compare: 'new' })).toBe('..new');
+  });
+
+  it('formats an empty range as empty so the param is stripped', () => {
+    expect(formatCompareRange({ base: '', compare: '' })).toBe('');
+  });
+});
+
+describe('resolveCompareRange', () => {
+  const revisions = [{ hash: 'new' }, { hash: 'mid' }, { hash: 'old' }];
+  const range = (base: string, compare: string): CompareRange => ({
+    base,
+    compare,
+  });
+
+  it('defaults compare to the head and base to the revision before it', () => {
+    expect(resolveCompareRange(revisions, range('', ''))).toEqual(
+      range('mid', 'new'),
+    );
+  });
+
+  it('keeps base one revision older than an explicit compare', () => {
+    expect(resolveCompareRange(revisions, range('', 'mid'))).toEqual(
+      range('old', 'mid'),
+    );
+  });
+
+  it('never defaults compare to the same revision as base', () => {
+    expect(resolveCompareRange(revisions, range('new', ''))).toEqual(
+      range('new', 'mid'),
+    );
+  });
+
+  it('leaves explicit hashes alone and copes with too few revisions', () => {
+    expect(resolveCompareRange(revisions, range('old', 'new'))).toEqual(
+      range('old', 'new'),
+    );
+    expect(resolveCompareRange([{ hash: 'only' }], range('', ''))).toEqual(
+      range('', 'only'),
+    );
+    expect(resolveCompareRange([], range('', ''))).toEqual(range('', ''));
+  });
+});
+
 describe('applyStatusPairFilter', () => {
   const rows = [
     { id: '1', sideA: 'PASS' as const, sideB: 'FAIL' as const },
diff --git a/dashboard/src/utils/treeCompareDiff.ts b/dashboard/src/utils/treeCompareDiff.ts
index cb313c7cd..e6e25ea59 100644
--- a/dashboard/src/utils/treeCompareDiff.ts
+++ b/dashboard/src/utils/treeCompareDiff.ts
@@ -28,7 +28,7 @@ export function apiStatusToItemStatus(
   return status;
 }
 
-/** Mirror backend _CHANGE_COUNT_SELECT categories for A→B transitions. */
+/** Mirror backend _CHANGE_COUNT_SELECT categories for base→compare transitions. */
 export function deriveCompareChange(
   statusA: CompareItemStatus,
   statusB: CompareItemStatus,
@@ -300,6 +300,40 @@ export function applyStatusPairFilter<
   );
 }
 
+export type CompareRange = { base: string; compare: string };
+
+/** `range` URL param is git-style `<base>..<compare>`; either side may be empty. */
+const COMPARE_RANGE_SEPARATOR = '..';
+
+export function parseCompareRange(range: string): CompareRange {
+  const [base = '', compare = ''] = range.split(COMPARE_RANGE_SEPARATOR, 2);
+  return { base, compare };
+}
+
+export function formatCompareRange({ base, compare }: CompareRange): string {
+  return base || compare ? `${base}${COMPARE_RANGE_SEPARATOR}${compare}` : '';
+}
+
+/**
+ * Fill in missing sides so the default comparison runs oldest → newest:
+ * compare defaults to the branch head, base to the revision right before it.
+ * `revisions` is newest-first.
+ */
+export function resolveCompareRange(
+  revisions: readonly { hash: string }[],
+  { base, compare }: CompareRange,
+): CompareRange {
+  const resolvedCompare =
+    compare || revisions.find(revision => revision.hash !== base)?.hash || '';
+  const compareIndex = revisions.findIndex(
+    revision => revision.hash === resolvedCompare,
+  );
+  return {
+    base: base || revisions[compareIndex + 1]?.hash || '',
+    compare: resolvedCompare,
+  };
+}
+
 /** Next/prev over the currently visible (searched/sorted) table rows. */
 export function compareRowNav<T extends { id: string }>(
   rows: T[],
diff --git a/docs/tree-compare.md b/docs/tree-compare.md
index ca455cdaa..73a9455a5 100644
--- a/docs/tree-compare.md
+++ b/docs/tree-compare.md
@@ -4,25 +4,25 @@ High-level overview of the Tree Compare feature: side-by-side comparison of buil
 
 ## Purpose
 
-Given a tree name and branch, pick two commit hashes (side A and side B) and answer:
+Given a tree name and branch, pick two commit hashes (base and compare) and answer:
 
-- How did overall pass/fail/inconclusive counts move from A → B?
+- How did overall pass/fail/inconclusive counts move from base → compare?
 - Which individual builds / boots / tests changed category (regressions, fixes, new failures, etc.)?
 
 Entry point: **Compare revisions** on Tree Details (`TreeCompareLink`), which opens:
 
-`/tree/{treeName}/{branch}/compare?hashA=…&hashB=…&origin=…`
+`/tree/{treeName}/{branch}/compare?range={baseHash}..{compareHash}&origin=…`
 
 ## User flow
 
-1. Open compare from Tree Details (current revision pre-fills as side A).
-2. Choose / swap revisions via the revision selector (commit history + shortcuts: previous commit, branch head, swap sides).
+1. Open compare from Tree Details (current revision pre-fills as **Compare**, **Base** defaults to the revision before it, so base → compare reads oldest → newest). The `range` URL param is git-style `<base>..<compare>`.
+2. Choose / swap revisions via the revision selector (commit history + shortcuts: previous commit, branch head, swap base and compare).
 3. Read the summary matrix (fixes, regressions, pass/fail/other counts per builds / boots / tests).
 4. Drill into **Changed results** tabs (Builds / Boots / Tests). Quick change-type chips add or remove their status pairs; custom From/To pairs can be added too (default: `PASS → FAIL` and `FAIL → PASS`). Last edited pairs are remembered in localStorage when the URL omits `statusPair`.
 
-URL search state owns: `hashA`, `hashB`, `origin`, `currentPageTab`, and optional `statusPair`.
+URL search state owns: `range` (`<base>..<compare>`), `origin`, `currentPageTab`, and optional `statusPair`.
 
-## Change categories (A → B)
+## Change categories (base → compare)
 
 Statuses are grouped into **PASS**, **FAIL**, and **INCONCLUSIVE** (everything else, including null). Absent on one side is treated as missing (`null` / `—`).
 
@@ -34,7 +34,7 @@ Statuses are grouped into **PASS**, **FAIL**, and **INCONCLUSIVE** (everything e
 | `stillFailing` | FAIL → FAIL |
 | `newPass` | missing/INCONCLUSIVE → PASS |
 | `appeared` | missing → INCONCLUSIVE |
-| `disappeared` | present on A, missing on B |
+| `disappeared` | present on base, missing on compare |
 | `unchanged` | same status on both sides (except FAIL → FAIL); frontend-only, has no backend count and no filter chip |
 
 Backend SQL aggregates (`_CHANGE_COUNT_SELECT` in `queries/tree.py`) and the frontend `deriveCompareChange` helper must stay in sync.