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When a \section starts at the top of a new page because the title and two lines (?) of text cannot fit at the bottom of the previous page, what is to take place on the previous in terms of paragraph spacing?

I'm using the book class and, when this occurs, the spacing between paragraphs on the previous page is increased rather than leaving the space at the bottom of the page.

An error of Underfull \vbox ... is returned and it took me quite awhile to realize that it was not due to the text I had just added to the document but what adding it caused in the next page in pushing the section forward and leaving "too much" vertical space on the previous page.

Would you please tell me what this is called such that I can locate it and learn what to do about it?

Thank you.

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    Welcome to TeX.SE! I think you can look up \flushbottom(used by the book class iirc) and \raggedbottom. Maybe you want the latter? With flushbottom, TeX will use the available stretch (if there is any) to "fill" the whole page. With raggedbottom, it will not.
    – mickep
    Commented Jun 23 at 8:21

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The \section code has no special branches detecting a page break or inserting anything in that case, so the behaviour is just down to the generic latex output routine (page breaker).

If there is not space on the page to break after the heading (because of penalties meaning there are no page break opportunities until a couple of lines after a heading) then TeX will try to break at before the vertical space above the heading, discarding that space and putting the heading at the top of the next page.

The remaining material will then be set to form the current page, to a vertical height of \textheight If \raggedbottom is in effect stretchable fil glue will be added at the end of the list before boxing the content.

TeX will then try to stretch the content to the specified height. With \raggedbottom that will always succeed, but with \flushbottom (the default for twocolumn or book) it will only succedd if there is existing stretchable glue on the page that can stretch enough, this may come from headings or the space around displays such as lists or displayed equations, or a strtechable \parskip for example.

If there is stretchable glue but it should not stretch that far you will get an underfull vbox warning with a badness indicating how much it is over stretched.

If there is no stretch glue, but is a breakpoint, the break will be infinitely bad which is cut off to an arbitrary badness of 10000 with an underfull vbox warning.

If there is no stretchable glue or breakpoint at all, the break won't happen so it will happen after the section and you will get an overfull box warning with the section extending in to the bottom margin.

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    is it really hbox not vbox?
    – user691586
    Commented Jun 23 at 19:13
  • @user691586 er no, fixed thanks. Commented Jun 23 at 20:41

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