diff --git a/user-doc/Intro.txt b/user-doc/Intro.txt
index 193eac81ab1af0dc3f8aae759c7b005a10edce61..d50e3f4d0cc7d106cd3020180bcc94d4d111be1c 100644
--- a/user-doc/Intro.txt
+++ b/user-doc/Intro.txt
@@ -83,6 +83,7 @@ PRINT ARG=d1 FILE=colvar STRIDE=10
 Also notice that all the actions can be labeled, and that many actions besides normal collective variables can define
 one or more value, which can be then referred using the corresponding label.
 
+\section Units A note on units
 By default the plumed inputs and outputs quantities in the following units:
 
 - Energy - kJ/mol