diff --git a/builtin-checkout.c b/builtin-checkout.c
index da04eed391..fe7c8584ca 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -689,7 +689,10 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	 * case 3: git checkout <something> [<paths>]
 	 *
 	 *   With no paths, if <something> is a commit, that is to
-	 *   switch to the branch or detach HEAD at it.
+	 *   switch to the branch or detach HEAD at it.  As a special case,
+	 *   if <something> is A...B (missing A or B means HEAD but you can
+	 *   omit at most one side), and if there is a unique merge base
+	 *   between A and B, A...B names that merge base.
 	 *
 	 *   With no paths, if <something> is _not_ a commit, no -t nor -b
 	 *   was given, and there is a tracking branch whose name is
@@ -715,7 +718,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		if (!strcmp(arg, "-"))
 			arg = "@{-1}";
 
-		if (get_sha1(arg, rev)) {
+		if (get_sha1_mb(arg, rev)) {
 			if (has_dash_dash)          /* case (1) */
 				die("invalid reference: %s", arg);
 			if (!patch_mode &&
diff --git a/cache.h b/cache.h
index 71a731dbc9..f8df15a060 100644
--- a/cache.h
+++ b/cache.h
@@ -703,6 +703,7 @@ extern const char *resolve_ref(const char *path, unsigned char *sha1, int, int *
 extern int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
 extern int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
 extern int interpret_branch_name(const char *str, struct strbuf *);
+extern int get_sha1_mb(const char *str, unsigned char *sha1);
 
 extern int refname_match(const char *abbrev_name, const char *full_name, const char **rules);
 extern const char *ref_rev_parse_rules[];
diff --git a/sha1_name.c b/sha1_name.c
index 44bb62d270..d5a240cf07 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -794,6 +794,48 @@ release_return:
 	return retval;
 }
 
+int get_sha1_mb(const char *name, unsigned char *sha1)
+{
+	struct commit *one, *two;
+	struct commit_list *mbs;
+	unsigned char sha1_tmp[20];
+	const char *dots;
+	int st;
+
+	dots = strstr(name, "...");
+	if (!dots)
+		return get_sha1(name, sha1);
+	if (dots == name)
+		st = get_sha1("HEAD", sha1_tmp);
+	else {
+		struct strbuf sb;
+		strbuf_init(&sb, dots - name);
+		strbuf_add(&sb, name, dots - name);
+		st = get_sha1(sb.buf, sha1_tmp);
+		strbuf_release(&sb);
+	}
+	if (st)
+		return st;
+	one = lookup_commit_reference_gently(sha1_tmp, 0);
+	if (!one)
+		return -1;
+
+	if (get_sha1(dots[3] ? (dots + 3) : "HEAD", sha1_tmp))
+		return -1;
+	two = lookup_commit_reference_gently(sha1_tmp, 0);
+	if (!two)
+		return -1;
+	mbs = get_merge_bases(one, two, 1);
+	if (!mbs || mbs->next)
+		st = -1;
+	else {
+		st = 0;
+		hashcpy(sha1, mbs->item->object.sha1);
+	}
+	free_commit_list(mbs);
+	return st;
+}
+
 /*
  * This is like "get_sha1_basic()", except it allows "sha1 expressions",
  * notably "xyz^" for "parent of xyz"
diff --git a/t/t2012-checkout-last.sh b/t/t2012-checkout-last.sh
index 87b30a268c..b44de9dc62 100755
--- a/t/t2012-checkout-last.sh
+++ b/t/t2012-checkout-last.sh
@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='checkout can switch to last branch'
+test_description='checkout can switch to last branch and merge base'
 
 . ./test-lib.sh
 
@@ -91,4 +91,29 @@ test_expect_success 'switch to twelfth from the last' '
 	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch13"
 '
 
+test_expect_success 'merge base test setup' '
+	git checkout -b another other &&
+	echo "hello again" >>world &&
+	git add world &&
+	git commit -m third
+'
+
+test_expect_success 'another...master' '
+	git checkout another &&
+	git checkout another...master &&
+	test "z$(git rev-parse --verify HEAD)" = "z$(git rev-parse --verify master^)"
+'
+
+test_expect_success '...master' '
+	git checkout another &&
+	git checkout ...master &&
+	test "z$(git rev-parse --verify HEAD)" = "z$(git rev-parse --verify master^)"
+'
+
+test_expect_success 'master...' '
+	git checkout another &&
+	git checkout master... &&
+	test "z$(git rev-parse --verify HEAD)" = "z$(git rev-parse --verify master^)"
+'
+
 test_done