diff --git a/README.md b/README.md
index 3f5b89237718b68a459e9d191e1b36078bc833e7..2c26cb29dedf7da958510c735cbac7f2f661ea23 100644
--- a/README.md
+++ b/README.md
@@ -94,6 +94,7 @@ Definitions:
 *_SHMS:  SHMS particle
 px,py,pz: paricle 3-momentum components in GeV
 E: particle Energy in GeV
+vz: z-vertex positin in cm (currently unused)
 weight: event weight
 ```
 Make sure you use the right coordinate system!