diff --git a/HACKING.md b/HACKING.md
index 0748517..4c8608a 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -21,4 +21,9 @@ is known to work flawlessly. Just do:
     $ awk -f template.awk template.html > template.h
 
 If your copy of `awk` is not the GNU implementation, you will need to
-install it and use `gawk` instead in the command line above.
+install it and use `gawk` instead in the command line above. 
+
+This includes macOS where the current built-in `awk` (currently version
+20070501 at time of testing on 10.13.6) doesn't apply correctly and causes
+characters to be omitted from the output. `gawk` can be installed with a 
+package manager such as [Homebrew](https://brew.sh).