diff --git a/linkcheck/checker/urlbase.py b/linkcheck/checker/urlbase.py
index 01b29e52..1971d86c 100644
--- a/linkcheck/checker/urlbase.py
+++ b/linkcheck/checker/urlbase.py
@@ -166,7 +166,11 @@ class UrlBase (object):
         # cache keys, are set by build_url() calling set_cache_keys()
         self.cache_url_key = None
         self.cache_content_key = None
-        # alias list
+        # Store a list of aliases since the same resource can be known
+        # under several URLs. This is used for example on HTTP 30[12]
+        # redirections.
+        # Note that aliases are saved as-is, not through the cache-key-
+        # generating method set_cache_keys().
         self.aliases = []
 
     def set_result (self, msg, valid=True):