The op­tional hy­phen, also known as the soft hy­phen, is usu­ally in­vis­i­ble. The op­tional hy­phen marks where a word should be hy­phen­ated if the word lands at the end of a line. You can put mul­ti­ple op­tional hy­phens in a word.

Why would you want to do this? Some words be­devil hy­phen­ation en­gines. For in­stance, True­Type will of­ten get hy­phen­ated as Tru-
eType, as in the ex­am­ple be­low, from a ri­val ty­pog­ra­phy book:

To pre­vent this, I put an op­tional hy­phen in the mid­dle (True~Type). Then it will be hy­phen­ated correctly.

How do you know if a word won’t be hy­phen­ated cor­rectly? The prob­lem usu­ally af­flicts words that aren’t in a stan­dard hy­phen­ation dic­tio­nary, like jar­gon words, un­usual proper names, and words with non­stan­dard spellings, like trade names. As Jus­tice Pot­ter Stew­art might have said, you’ll know it when you see it.