diff --git a/mdevaluate/pbc.py b/mdevaluate/pbc.py
index 99ee9f0..b851e67 100644
--- a/mdevaluate/pbc.py
+++ b/mdevaluate/pbc.py
@@ -252,6 +252,8 @@ def pbc_points(coordinates, box, thickness=0, index=False, shear=False):
     """
     if shear:
         box[2, 0] = box[2, 0] % box[2, 2]
+        # Shifts the box images in the other directions if they moved more than
+        # half the box length
         if box[2, 0] > box[2, 2]:
             box[2, 0] = box[2, 0] - box[2, 2]