# Why setting 'predisplaypenalty' to 0 is wrong idea?

In Vertical space between section header and text is too big user egreg said that setting \predisplaypenalty=0 is wrong idea.

Why is that? Could you provide an example?

If not '0', what value could be used?

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The parameter \predisplaypenalty is the "cost" that TeX assigns to inserting a page break immediately before the start of a displayed equation (or multiple equations). On p. 189 of the TeXbook, Knuth writes:
Plain TeX sets \predisplaypenalty=10000, because fine printers traditionally shun displayed formulas at the very top of a page."
[Asides: (i) If a TeX penalty parameter is set to 10,000, it's the functional equivalent of infinity. (ii) LaTeX also sets \predisplaypenalty=10000.] If one were to set \predisplaypenalty=0, this cost parameter would be 0, i.e., TeX wouldn't even try to find alternative page break points than the one it may have "found" immediately before the start of a displayed equation.