diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -4914,7 +4914,7 @@
     overrides = {(b'ui', b'forcemerge'): opts.get(b'tool', b'')}
     with ui.configoverride(overrides, b'merge'):
         force = opts.get(b'force')
-        labels = [b'working copy', b'merge rev']
+        labels = [b'working copy', b'merge rev', b'common ancestor']
         return hg.merge(ctx, force=force, labels=labels)
 
 
diff --git a/tests/test-annotate.t b/tests/test-annotate.t
--- a/tests/test-annotate.t
+++ b/tests/test-annotate.t
@@ -221,16 +221,16 @@
   a
   a
   a
-  <<<<<<< working copy: 5fbdc1152d97 - test: b2.1
+  <<<<<<< working copy:    5fbdc1152d97 - test: b2.1
   b4
   c
   b5
-  ||||||| base:         3086dbafde1c - test: b
+  ||||||| common ancestor: 3086dbafde1c - test: b
   =======
   b4
   b5
   b6
-  >>>>>>> merge rev:    37ec9f5c3d1f - test: b2
+  >>>>>>> merge rev:       37ec9f5c3d1f - test: b2
   $ cat <<EOF > b
   > a
   > a
@@ -747,16 +747,16 @@
   0
   1 baz:1
   2 baz:2
-  <<<<<<< working copy: 863de62655ef - test: baz:3+->3-
+  <<<<<<< working copy:    863de62655ef - test: baz:3+->3-
   3- baz:3
   4 baz:4
-  ||||||| base:         56fc739c091f - test: baz:3->3+
+  ||||||| common ancestor: 56fc739c091f - test: baz:3->3+
   3+ baz:3
   4 baz:4
   =======
   3+ baz:3
   4+ baz:4
-  >>>>>>> merge rev:    cb8df70ae185 - test: qux:4->4+
+  >>>>>>> merge rev:       cb8df70ae185 - test: qux:4->4+
   5
   6
   7
@@ -794,16 +794,16 @@
   0
   1 baz:1
   2 baz:2
-  <<<<<<< working copy: cb8df70ae185 - test: qux:4->4+
+  <<<<<<< working copy:    cb8df70ae185 - test: qux:4->4+
   3+ baz:3
   4+ baz:4
-  ||||||| base:         56fc739c091f - test: baz:3->3+
+  ||||||| common ancestor: 56fc739c091f - test: baz:3->3+
   3+ baz:3
   4 baz:4
   =======
   3- baz:3
   4 baz:4
-  >>>>>>> merge rev:    863de62655ef - test: baz:3+->3-
+  >>>>>>> merge rev:       863de62655ef - test: baz:3+->3-
   5
   6
   7
@@ -1172,16 +1172,16 @@
   warning: conflicts while merging a! (edit, then use 'hg resolve --mark')
   [1]
   $ cat a
-  <<<<<<< working copy: 0a068f0261cf - test: 3
+  <<<<<<< working copy:    0a068f0261cf - test: 3
   1
   2
   3
-  ||||||| base:         1ed24be7e7a0 - test: 2
+  ||||||| common ancestor: 1ed24be7e7a0 - test: 2
   1
   2
   =======
   a
-  >>>>>>> merge rev:    9409851bc20a - test: a
+  >>>>>>> merge rev:       9409851bc20a - test: a
   $ cat > a << EOF
   > b
   > 1
diff --git a/tests/test-conflict.t b/tests/test-conflict.t
--- a/tests/test-conflict.t
+++ b/tests/test-conflict.t
@@ -267,7 +267,7 @@
   3
   6
   8
-  ||||||| base
+  ||||||| common ancestor
   One
   Two
   Three
diff --git a/tests/test-fastannotate-hg.t b/tests/test-fastannotate-hg.t
--- a/tests/test-fastannotate-hg.t
+++ b/tests/test-fastannotate-hg.t
@@ -165,16 +165,16 @@
   a
   a
   a
-  <<<<<<< working copy: 5fbdc1152d97 - test: b2.1
+  <<<<<<< working copy:    5fbdc1152d97 - test: b2.1
   b4
   c
   b5
-  ||||||| base:         3086dbafde1c - test: b
+  ||||||| common ancestor: 3086dbafde1c - test: b
   =======
   b4
   b5
   b6
-  >>>>>>> merge rev:    37ec9f5c3d1f - test: b2
+  >>>>>>> merge rev:       37ec9f5c3d1f - test: b2
   $ cat <<EOF > b
   > a
   > a
@@ -789,16 +789,16 @@
   warning: conflicts while merging a! (edit, then use 'hg resolve --mark')
   [1]
   $ cat a
-  <<<<<<< working copy: 0a068f0261cf - test: 3
+  <<<<<<< working copy:    0a068f0261cf - test: 3
   1
   2
   3
-  ||||||| base:         1ed24be7e7a0 - test: 2
+  ||||||| common ancestor: 1ed24be7e7a0 - test: 2
   1
   2
   =======
   a
-  >>>>>>> merge rev:    9409851bc20a - test: a
+  >>>>>>> merge rev:       9409851bc20a - test: a
   $ cat > a << EOF
   > b
   > 1
diff --git a/tests/test-merge-changedelete.t b/tests/test-merge-changedelete.t
--- a/tests/test-merge-changedelete.t
+++ b/tests/test-merge-changedelete.t
@@ -680,12 +680,12 @@
   changed
   --- file3 ---
   3
-  <<<<<<< working copy: 13910f48cf7b - test: changed file1, removed file2, chan...
+  <<<<<<< working copy:    13910f48cf7b - test: changed file1, removed file2, c...
   changed2
-  ||||||| base:         ab57bf49aa27 - test: added files
+  ||||||| common ancestor: ab57bf49aa27 - test: added files
   =======
   changed1
-  >>>>>>> merge rev:    10f9a0a634e8 - test: removed file1, changed file2, chan...
+  >>>>>>> merge rev:       10f9a0a634e8 - test: removed file1, changed file2, c...
 
 Exercise transitions between local, other, fail and prompt, and make sure the
 dirstate stays consistent. (Compare with each other and to the above
diff --git a/tests/test-merge-force.t b/tests/test-merge-force.t
--- a/tests/test-merge-force.t
+++ b/tests/test-merge-force.t
@@ -398,13 +398,13 @@
   content2
   
   M content1_content2_content1_content4-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         8ef80617fa20 - test: base
+  ||||||| common ancestor: 8ef80617fa20 - test: base
   content1
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M content1_content2_content1_content4-untracked
   content2
@@ -428,13 +428,13 @@
   content2
   
   M content1_content2_content2_content4-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         8ef80617fa20 - test: base
+  ||||||| common ancestor: 8ef80617fa20 - test: base
   content1
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M content1_content2_content2_content4-untracked
   content2
@@ -458,25 +458,25 @@
   content2
   
   M content1_content2_content3_content3-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content3
-  ||||||| base:         8ef80617fa20 - test: base
+  ||||||| common ancestor: 8ef80617fa20 - test: base
   content1
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M content1_content2_content3_content3-untracked
   content2
   
   M content1_content2_content3_content4-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         8ef80617fa20 - test: base
+  ||||||| common ancestor: 8ef80617fa20 - test: base
   content1
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M content1_content2_content3_content4-untracked
   content2
@@ -500,13 +500,13 @@
   content2
   
   M content1_content2_missing_content4-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         8ef80617fa20 - test: base
+  ||||||| common ancestor: 8ef80617fa20 - test: base
   content1
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M content1_content2_missing_content4-untracked
   content2
@@ -584,12 +584,12 @@
   content2
   
   M missing_content2_content2_content4-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         000000000000 - : 
+  ||||||| common ancestor: 000000000000 - : 
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M missing_content2_content2_content4-untracked
   content2
@@ -607,23 +607,23 @@
   content2
   
   M missing_content2_content3_content3-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content3
-  ||||||| base:         000000000000 - : 
+  ||||||| common ancestor: 000000000000 - : 
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M missing_content2_content3_content3-untracked
   content2
   
   M missing_content2_content3_content4-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         000000000000 - : 
+  ||||||| common ancestor: 000000000000 - : 
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M missing_content2_content3_content4-untracked
   content2
@@ -641,20 +641,20 @@
   content2
   
   M missing_content2_missing_content4-tracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         000000000000 - : 
+  ||||||| common ancestor: 000000000000 - : 
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M missing_content2_missing_content4-untracked
-  <<<<<<< working copy: 0447570f1af6 - test: local
+  <<<<<<< working copy:    0447570f1af6 - test: local
   content4
-  ||||||| base:         000000000000 - : 
+  ||||||| common ancestor: 000000000000 - : 
   =======
   content2
-  >>>>>>> merge rev:    85100b8c675b - test: remote
+  >>>>>>> merge rev:       85100b8c675b - test: remote
   
   M missing_content2_missing_missing-tracked
   content2
diff --git a/tests/test-merge-tools.t b/tests/test-merge-tools.t
--- a/tests/test-merge-tools.t
+++ b/tests/test-merge-tools.t
@@ -1223,15 +1223,15 @@
   # hg update -C 1
   $ hg merge -r 4 --config merge-tools.true.premerge=keep-merge3
   merging f
-  <<<<<<< working copy: ef83787e2614 - test: revision 1
+  <<<<<<< working copy:    ef83787e2614 - test: revision 1
   revision 1
   space
-  ||||||| base:         ffd2bda21d6e - test: revision 0
+  ||||||| common ancestor: ffd2bda21d6e - test: revision 0
   revision 0
   space
   =======
   revision 4
-  >>>>>>> merge rev:    81448d39c9a0 - test: revision 4
+  >>>>>>> merge rev:       81448d39c9a0 - test: revision 4
   revision 0
   space
   revision 4
@@ -1239,15 +1239,15 @@
   (branch merge, don't forget to commit)
   $ aftermerge
   # cat f
-  <<<<<<< working copy: ef83787e2614 - test: revision 1
+  <<<<<<< working copy:    ef83787e2614 - test: revision 1
   revision 1
   space
-  ||||||| base:         ffd2bda21d6e - test: revision 0
+  ||||||| common ancestor: ffd2bda21d6e - test: revision 0
   revision 0
   space
   =======
   revision 4
-  >>>>>>> merge rev:    81448d39c9a0 - test: revision 4
+  >>>>>>> merge rev:       81448d39c9a0 - test: revision 4
   # hg stat
   M f
   # hg resolve --list
@@ -1264,12 +1264,12 @@
   $ hg merge -r 4 --config merge-tools.true.premerge=keep-mergediff
   merging f
   <<<<<<<
-  ------- base:         ffd2bda21d6e - test: revision 0
-  +++++++ working copy: ef83787e2614 - test: revision 1
+  ------- common ancestor: ffd2bda21d6e - test: revision 0
+  +++++++ working copy:    ef83787e2614 - test: revision 1
   -revision 0
   +revision 1
    space
-  ======= merge rev:    81448d39c9a0 - test: revision 4
+  ======= merge rev:       81448d39c9a0 - test: revision 4
   revision 4
   >>>>>>>
   revision 0
@@ -1280,12 +1280,12 @@
   $ aftermerge
   # cat f
   <<<<<<<
-  ------- base:         ffd2bda21d6e - test: revision 0
-  +++++++ working copy: ef83787e2614 - test: revision 1
+  ------- common ancestor: ffd2bda21d6e - test: revision 0
+  +++++++ working copy:    ef83787e2614 - test: revision 1
   -revision 0
   +revision 1
    space
-  ======= merge rev:    81448d39c9a0 - test: revision 4
+  ======= merge rev:       81448d39c9a0 - test: revision 4
   revision 4
   >>>>>>>
   # hg stat
@@ -1592,7 +1592,7 @@
   arg: "ll:working copy"
   arg: "lo:"
   arg: "merge rev"
-  arg: "lb:base: */f~base.*" (glob)
+  arg: "lb:common ancestor: */f~base.*" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'
@@ -1619,7 +1619,7 @@
   arg: "ll:working copy"
   arg: "lo:"
   arg: "merge rev"
-  arg: "lb:base: */hgmerge.*/f~base" (glob)
+  arg: "lb:common ancestor: */hgmerge.*/f~base" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'
@@ -1649,7 +1649,7 @@
   arg: "ll:working copy: tooltmpl ef83787e2614"
   arg: "lo:"
   arg: "merge rev: tooltmpl 0185f4e0cf02"
-  arg: "lb:base: */f~base.*" (glob)
+  arg: "lb:common ancestor: */f~base.*" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'
@@ -2015,7 +2015,7 @@
   Running merge tool for b ("*/bin/echo.exe"): (glob) (windows !)
   Running merge tool for b (*/bin/echo): (glob) (no-windows !)
   - local (working copy): 10:2d1f533d add binary file (#2) tip default
-  -          base (base): -1:00000000  default
+  - base (common ancestor): -1:00000000  default
   -    other (merge rev): 9:1e7ad7d7 add binary file (#1) default
   merge runs here ...
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
diff --git a/tests/test-sparse.t b/tests/test-sparse.t
--- a/tests/test-sparse.t
+++ b/tests/test-sparse.t
@@ -211,11 +211,11 @@
   hide
 
   $ cat hide
-  <<<<<<< dest:             39278f7c08a9 - test: two
+  <<<<<<< dest:   39278f7c08a9 - test: two
   y
   =======
   z
-  >>>>>>> source:           b91df4f39e75 - test: edit hide
+  >>>>>>> source: b91df4f39e75 - test: edit hide
 
 Verify aborting a rebase cleans up temporary files