diff --git a/Trajectories/extract_traj.py b/Trajectories/extract_traj.py
index 43c406671cae7f9419e511b2b93e20b2b46355ed..958a530ea7ba10b610cb9936fbc48a604b3bbd7b 100755
--- a/Trajectories/extract_traj.py
+++ b/Trajectories/extract_traj.py
@@ -1,5 +1,11 @@
 #!/usr/bin/env python3
 
+"""This script reads a JSon file containing trajectories as lists of
+instantaneous eddies and prints the trajectory which has the given
+index.
+
+"""
+
 import json
 import sys