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I've been searching how to center a piece of text vertically on a page, since I wanted to create a cover for my reports with the Title centered both vertically and horizontally. I first tried the following code:

\vfil
\centerline{Title}
\vfil

\eject\end

But this isn't centered vertically. So I found this answer, and so I added \leavevmode to the beginning of the file. And it worked! I also tried putting instead \indentat the beginning of the original. And it also worked!

So my question is: Why do you need to start a paragraph or "leave vertical mode" before the first \vfil in order for the glue to work in centering the text veertically on the page? It all seems very odd, indeed.

tigre200
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    Because glue is discardable at a page break. p112 of The TeXBook explains the mechanism. – Thruston Jan 21 '19 at 17:28
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    You can also use \null or \hbox{} or... many other options. The reason/idea of discarding glue at the top of a page is similar to discarding glue at the start of a line: after a line/page break, you don't want “leftover” glue from the previous line/page to affect the current one. – ShreevatsaR Jan 21 '19 at 18:46
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    @JohnKormylo -- This question is about plain tex. \vspace is latex only. – barbara beeton Apr 10 '19 at 01:07

1 Answers1

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As mentioned in the comments \TeX ignores glue at the end or top of a page as it does ignore glue at the end or start of a line.

But plain \TeX knows some commands for glue that does not vanish; you do not need to enter a special command like \leavevmode. For glue at the top of a page \topglue is used instead of \topskip. (And there is \hglue and \vglue as non-discarding variants of \hskip and \vskip.)

For your problem this \topglue provides a solution. As \bye contains \vfill, i.e., glue with specification 0pt plus 1fill, input

\topglue 0pt plus 1fill
\centerline{Titel}
\bye
Udo Wermuth
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