diff --git a/t/t3427-rebase-subtree.sh b/t/t3427-rebase-subtree.sh
index d8640522a0..3a2ae7b55d 100755
--- a/t/t3427-rebase-subtree.sh
+++ b/t/t3427-rebase-subtree.sh
@@ -11,6 +11,34 @@ commit_message() {
 	git log --pretty=format:%s -1 "$1"
 }
 
+# There are a few bugs in the rebase with regards to the subtree strategy, and
+# this test script tries to document them.  First, the following commit history
+# is generated (the onelines are shown, time flows from left to right):
+#
+# master1 - master2 - master3
+#                             \
+# README ---------------------- Add subproject master - master4 - files_subtree/master5
+#
+# Where the merge moves the files master[123].t into the subdirectory
+# files_subtree/ and master4 as well as files_subtree/master5 add files to that
+# directory directly.
+#
+# Then, in subsequent test cases, `git filter-branch` is used to distill just
+# the commits that touch files_subtree/. To give it a final pre-rebase touch,
+# an empty commit is added on top. The pre-rebase commit history looks like
+# this:
+#
+# Add subproject master - master4 - files_subtree/master5 - Empty commit
+#
+# where the root commit adds three files: master1.t, master2.t and master3.t.
+#
+# This commit history is then rebased onto `master3` with the
+# `-Xsubtree=files_subtree` option in three different ways:
+#
+# 1. using `--preserve-merges`
+# 2. using `--preserve-merges` and --keep-empty
+# 3. without specifying a rebase backend
+
 test_expect_success 'setup' '
 	test_commit README &&
 	mkdir files &&