5.28.1.2 The Implicit Page Trap

If, after starting GNU troff without loading a macro package, you use the ptr request to dump a list of the active traps to the standard error stream,¹⁰⁹ nothing is reported. Yet the .t register will report a steadily decreasing value with every output line your document produces, and once the value of .t gets to within .V of zero, you will notice that something trap-like happens—the page is ejected, a new one begins, and the value of .t becomes large once more.

This implicit page trap always exists in the top-level diversion;¹¹⁰ it works like a trap in some ways but not others. Its purpose is to eject the current page and start the next one. It has no name, so it cannot be moved or deleted with wh or ch requests. You cannot hide it by placing another trap at its location, and can move it only by redefining the page length with pl. Its operation is suppressed when vertical page traps are disabled with GNU troff’s vpt request.