diff --git a/Trajectories/trajectories.py b/Trajectories/trajectories.py
index 63a1ccab25fa9b2e9642e52ea3c653e80bdb9332..f8e1a81c81db94cbb09d171f2fd5f6186a7f6058 100755
--- a/Trajectories/trajectories.py
+++ b/Trajectories/trajectories.py
@@ -7,6 +7,9 @@ Input: the graph of segments produced by Graph-tool (in gt or graphml
 format). Output: trajectories as lists of segments and as lists of
 instantaneous eddies, in JSon files.
 
+By construction, the segments and the instantaneous eddies in each
+trajectory are in ascending order.
+
 """
 
 import graph_tool