diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 41f50dc..cd09a1a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -2,9 +2,18 @@
 
 This project is formatted with [clang-format][fmt] using the style file at the root of the repository. Please run clang-format before sending a pull request.
 
-In general, try to follow the style of surrounding code.
+In general, try to follow the style of surrounding code. We mostly follow the [Google C++ style guide][cpp-style].
+
+Commit messages should be in the imperative mood, as described in the [Git contributing file][git-contrib]:
+
+> Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
+> instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
+> to do frotz", as if you are giving orders to the codebase to change
+> its behaviour.
 
 [fmt]: http://clang.llvm.org/docs/ClangFormat.html
+[cpp-style]: https://google.github.io/styleguide/cppguide.html
+[git-contrib]: http://git.kernel.org/cgit/git/git.git/tree/Documentation/SubmittingPatches?id=HEAD
 
 # Tests