--- a/devel-versioning.rst	Sun Sep 25 16:08:12 2011 +0300
+++ b/devel-versioning.rst	Sun Sep 25 16:31:40 2011 +0300
@@ -161,6 +161,22 @@
 
   GLib GTK+ Gimp GNOME Kaffe
 
+Forms of compatibility.
+=======================
+
+Runtime of binary compatibility mean that binary can be swaped with another
+version without breaking.
+
+Compile time or source compatibility mean that supplied header/interfaces allow
+successful compiling/linking.
+
+Example:
+
+ * Change type of argument in method to more generic take source compatibility
+   but break binary compatibility.
+ * Adding new mothod to abstract class take binary compatibility but break
+   compilation (unimplement method error).
+
 Compatibility formula.
 ======================