Vertical Position Traps (The GNU Troff Manual)

#### 5.28.1 Vertical Position Traps

A vertical position trap calls a macro when the formatter’s vertical drawing position reaches or passes, in the downward direction, a certain location on the output page or in a diversion. Its 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; see The Implicit Page Trap.