Keep lines to­gether en­sures that all lines in a para­graph ap­pear on the same page. If the last line of the para­graph won’t fit on the cur­rent page, the whole para­graph will be moved to the next page.

Use this op­tion with head­ings to pre­vent them from start­ing at the bot­tom of one page and con­tin­u­ing at the top of the next. That looks bad.

Like widow and or­phan con­trol, keep­ing lines to­gether will cre­ate gaps at the bot­tom of pages. But un­like widow and or­phan con­trol, you only want to keep lines to­gether in spe­cial sit­u­a­tions, not as part of your de­fault text formatting.

Why? Keep­ing lines to­gether is a blunter tech­nique. It only works on whole para­graphs, so the longer the para­graph, the big­ger the gap.

If you need to make groups of el­e­ments stick to­gether, the keep-lines-to­gether op­tion works well with hard line breaks. Re­call that hard line breaks don’t cre­ate a new para­graph, but rather a set of lines. Keep­ing lines to­gether will en­sure this set of lines ap­pears on a sin­gle page.

For in­stance, it’s help­ful to keep lines to­gether in sig­na­ture blocks:

Here, you’d put a hard line break at the end of each line. Then you’d set the whole sig­na­ture block—which is a sin­gle para­graph—to keep lines to­gether. That way, you’ll never have to worry that half the block will end up on one page and half on the next. All of it trav­els together.