--- a/devel-versioning.rst	Mon Nov 28 01:32:11 2011 +0200
+++ b/devel-versioning.rst	Mon Nov 28 01:32:24 2011 +0200
@@ -346,6 +346,23 @@
 Forms of compatibility.
 =======================
 
+Backward/forward compatibility.
+-------------------------------
+
+Backward compatibility for library, file format, protocol means that new version
+of program can work with old library, file format, protocol.
+
+Forward compatibility for library, file format, protocol means that old version
+of program can work with new library, file format, protocol but without using
+any benefits from new versions and more essentially without posibility damage
+any user data.
+
+Example of backward compatibility: adding to graphic library new image format
+for reading/saving.
+
+Example of forward compatibility: old browser ignore any new (unknown) HTML
+tags.
+
 Source/binary compatibility.
 ----------------------------