diff --git a/doc/contributing.texi b/doc/contributing.texi
index ff7065ad2a..18f3705a43 100644
--- a/doc/contributing.texi
+++ b/doc/contributing.texi
@@ -563,7 +563,9 @@ Source Tree Structure
 
 @table @file
 @item guix
-This is the location of core Guix mechanisms.  A few examples:
+This is the location of core Guix mechanisms.  To illustrate what is
+meant by ``core'', here are a few examples, starting from low-level
+tools and going towards higher-level tools:
 
 @table @code
 @item (guix store)
@@ -638,10 +640,16 @@ Source Tree Structure
 @end table
 
 The directories we have seen so far all live under @file{guix/}.  The
-other important place is the @code{gnu/} directory, which contains
+other important place is the @file{gnu/} directory, which contains
 primarily package definitions as well as libraries and tools for Guix
 System (@pxref{System Configuration}) and Guix Home (@pxref{Home
-Configuration}).
+Configuration}), all of which build upon functionality provided by
+@code{(guix @dots{})} modules@footnote{For this reason, @code{(guix
+@dots{})}  modules must generally not depend on @code{(gnu @dots{})}
+modules, with one notable exception: @code{(guix build-system @dots{})}
+modules may look up packages at run time---e.g., @code{(guix
+build-system cmake)} needs to access the @code{cmake} variable at run
+time.}.
 
 @table @file
 @cindex package modules