diff --git a/README.devel b/README.devel
index 2bd33bb815225cca5edac643fc91429de5892960..9d736185f1bdb323e6f95b6ff397eca37452cc18 100644
--- a/README.devel
+++ b/README.devel
@@ -16,7 +16,8 @@ post-processing view-dependent option.
 to access it), a reference to a handling routine (i.e. opt_XXX) and a
 default value for this option.
 
-3) Create the handling routine opt_XXX in Common/Options.cpp.
+3) Create the handling routine opt_XXX in Common/Options.cpp (and add
+the prototype in Common/Options.h).
 
 4) Create the associated widget in Fltk/GUI.cpp