What does the actual phrase above mean?


99 times out of 100 it means you have \\ incorrectly placed at the end of a paragraph. But to dissect the message:

Underfull \hbox (badness 10000) in paragraph at lines 4--5

A box is underfull if there is not enough content to fill its stated size.

If you go \hbox to 5cm{A} then it makes a horizontal box (\hbox) 5cm wide just containing an A so it is underfull and will generate a warning, the exact amount of badness depends how much any white space is over-stretched, but here there is no white space so it is infinitely bad, which is arbitrarily truncated to the maximum value, 10000.

By placing \\ at the end of a paragraph you force a line break but there is nothing at all in the forced final line of the paragraph so it is a box that is \textwidth wide with no content. It appears a bit like vertical space but it is not it is a spurious line at the end of the paragraph. So for example it does not stretch and is not dropped at the start of a page.

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  • @gernot oh I was out by a factor of 10, thanks:-) – David Carlisle Oct 15 '16 at 16:43
  • Thank you for that well and clearly explained answer to my question. – Charles Boehm Oct 16 '16 at 9:46
  • @CharlesBoehm Can you please explain this to me ? – user2925716 Oct 8 '19 at 19:11
  • What if I write \\ , put a blank line and then write something?. In this scenario, there is something written after \\, but I am still getting this error! – sreeraj t Mar 31 at 4:05
  • 1
    @sreerajt that is exactly the case described here: it is always wrong to have a blank line after \\ – David Carlisle Mar 31 at 7:48

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