diff --git a/sohstationviewer/documentation/13 _ Ruler and Zooming.help.md b/sohstationviewer/documentation/13 _ Ruler and Zooming.help.md
index daf89a5281dadad3e9c7cf685a1ff5df18ff65a0..768e4c575aaf6b4baed8b1827224b2935d5af610 100644
--- a/sohstationviewer/documentation/13 _ Ruler and Zooming.help.md	
+++ b/sohstationviewer/documentation/13 _ Ruler and Zooming.help.md	
@@ -16,7 +16,9 @@ all the TPS channels. Ctr/cmd + Click on TPS plot will work the same.
 ---------------------------
 
 # Zooming
-Shift + Click to mark the first zooming point, then Shift + click again to mark 
+
+## Zoom in
+Shift + Click on a plot to mark the first zooming point, then Shift + click again to mark 
 the second zooming point to zoom the area between two zooming points.
 
 The zooming task will be performed in the area corresponding to zooming triggering 
@@ -31,3 +33,7 @@ be triggered from TPS dialog.
 <br />
 <br />
 <br />
+
+## Zoom out
+Shift + Click on the left side of a plot to zoom out one level, which means
+undoing one zoom in. 
\ No newline at end of file