diff --git a/segments.py b/segments.py
index 9b98808ea41f4626a2dd20434c9a2547c4b9ba9c..1b6f561133c9280c84828e61abf10d5b5d18e00f 100755
--- a/segments.py
+++ b/segments.py
@@ -2,7 +2,8 @@
 
 """Collapses the input graph into a new graph: the graph of
 segments. An edge of the graph is a segment. The new graph is output
-in the binary format of graph-tool.
+in the binary format of graph-tool. Input file should be an edgelist
+in CSV format. Output file can have suffix ".gt" or ".graphml".
 
 """