A sig­na­ture line is a hor­i­zon­tal line aligned with ad­ja­cent text.

Ty­pog­ra­phy purists avoid ac­com­plish­ing any­thing by hold­ing down keys on the key­board. But in this case it’s the sim­plest so­lu­tion. To make a sig­na­ture line, hold down the un­der­score key (shift + hy­phen) un­til you get the length you need.

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A ri­val school of thought sug­gests you should type a se­ries of word spaces and for­mat them with un­der­lin­ing.

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Same thing, right? Not quite. There are three good rea­sons to pre­fer un­der­scores to un­der­lined word spaces.

1. If you need to quickly rid a doc­u­ment of un­der­lin­ing, you might want to se­lect all the text and then uncheck the un­der­lin­ing op­tion. But this will wreck sig­na­ture lines made out of un­der­lined word spaces—they will disappear.

2. If you need to quickly en­sure you only have one space be­tween sen­tences, you might want to search for and re­place any dou­ble spaces. But this will also wreck sig­na­ture lines made out of word spaces—by par­tially delet­ing them.

3. Un­der­score char­ac­ters don’t de­pend on for­mat­ting, so they will look the same no mat­ter where they’re copied and pasted. Un­der­lined word spaces may not.

You should de­part from this rule only if the font you’re us­ing has an un­der­score char­ac­ter that doesn’t form a solid line when used in se­quence. That’s how un­der­scores are sup­posed to work, but some fonts are un­co­op­er­a­tive. If you see gaps be­tween the un­der­scores, ei­ther use un­der­lined word spaces, or use the un­der­scores from a dif­fer­ent font. No one will know.