diff --git a/propka/atom.py b/propka/atom.py
index 5db56e3..cd399b1 100644
--- a/propka/atom.py
+++ b/propka/atom.py
@@ -2,7 +2,6 @@
 import string
 import propka.lib
 import propka.group
-# TODO - what is hybrid36?
 from . import hybrid36
 
 
diff --git a/propka/hybrid36.py b/propka/hybrid36.py
index f5b96af..e0af2dc 100644
--- a/propka/hybrid36.py
+++ b/propka/hybrid36.py
@@ -1,3 +1,7 @@
+"""Provides an alternative PDB format that can transparently encode larger atom numbers.
+
+http://cci.lbl.gov/hybrid_36/
+"""
 import string
 
 _hybrid36_upper_chars = set(string.ascii_uppercase)