diff --git a/HACKING b/HACKING
index cc4983f6..03605dd6 100644
--- a/HACKING
+++ b/HACKING
@@ -10,7 +10,9 @@ Coding Style
 ------------
 
 1.  All code should be indented with tabs. (Ignore the use of only spaces in
-    this file) By default, source files contain the following Vim modeline:
+    this file.) A tab size of two spaces is used when calculating line widths,
+    which should be a maximum of 80 characters. By default, source files
+    contain the following Vim modeline:
 +
 [source,C]
 -------------------------------------------