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-rw-r--r--Documentation/SubmittingPatches29
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diff --git a/Documentation/SubmittingPatches b/Documentation/SubmittingPatches
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--- a/Documentation/SubmittingPatches
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@@ -110,6 +110,35 @@ run `git diff --check` on your changes before you commit.
[[describe-changes]]
=== Describe your changes well.
+The log message that explains your changes is just as important as the
+changes themselves. Your code may be clearly written with in-code
+comment to sufficiently explain how it works with the surrounding
+code, but those who need to fix or enhance your code in the future
+will need to know _why_ your code does what it does, for a few
+reasons:
+
+. Your code may be doing something differently from what you wanted it
+ to do. Writing down what you actually wanted to achieve will help
+ them fix your code and make it do what it should have been doing
+ (also, you often discover your own bugs yourself, while writing the
+ log message to summarize the thought behind it).
+
+. Your code may be doing things that were only necessary for your
+ immediate needs (e.g. "do X to directories" without implementing or
+ even designing what is to be done on files). Writing down why you
+ excluded what the code does not do will help guide future developers.
+ Writing down "we do X to directories, because directories have
+ characteristic Y" would help them infer "oh, files also have the same
+ characteristic Y, so perhaps doing X to them would also make sense?".
+ Saying "we don't do the same X to files, because ..." will help them
+ decide if the reasoning is sound (in which case they do not waste
+ time extending your code to cover files), or reason differently (in
+ which case, they can explain why they extend your code to cover
+ files, too).
+
+The goal of your log message is to convey the _why_ behind your
+change to help future developers.
+
The first line of the commit message should be a short description (50
characters is the soft limit, see DISCUSSION in linkgit:git-commit[1]),
and should skip the full stop. It is also conventional in most cases to