5.28.1 Vertical Position Traps

Vertical position traps perform an action when GNU troff reaches or passes a certain vertical location on the output page or in a diversion. Their applications include setting page headers and footers, body text in multiple columns, and footnotes.

Request: .vpt [flag]
Register: \n[.vpt]

Enable vertical position traps if flag is non-zero or absent; disable them otherwise. Vertical position traps are those set by the wh request or by dt within a diversion. The parameter that controls whether vertical position traps are enabled is global. Initially, vertical position traps are enabled. The current value is stored in the .vpt read-only register.

A page can’t be ejected if vpt is set to zero.