# em, ex, en? Which length units are font size dependent?

Length can be specified as 3em, or 3.5ex. What is required to make 4en also valid? Are there any other font related length units?

• What's the use? 1en=0.5em. – Bernard Aug 15 '16 at 9:43
• and if I wanted \aye for the width of an i? – Yossi Gil Aug 15 '16 at 9:46
• Why not \fjord for the width of the ligature fj;o)? – Bernard Aug 15 '16 at 9:53
• em is not the width of a character m!!! so your comment about the width of an i seems misplaced, you can use \fontcharwd to find the width of a character in a font. – David Carlisle Aug 15 '16 at 9:55

There is no en unit. TFM fonts have font dimensions 5 and 6 which may be accessed as ex and em no other fontdimen corresponds directly to a unit.
Note in comments you ask about the width of an i you can use e-tex \fontcharwd to find the width of a character or classically \setbox0\hbox{i}... \wd0 But this em and ex are not related to the width or height of the m and x characters they are just arbitrary lengths assigned by the font designer.
• But there is an \enspace macro, which has sometimes lured me into thinking en is defined as a unit in LaTeX. – alephzero Aug 15 '16 at 12:58
• there's a \bigskipamount as well but that isn't a unit either, – David Carlisle Aug 15 '16 at 12:59
• True enough, but en is (or was, historically) a common printing term - often called a "nut" to distinguish it clearly from a "mutton" (em) when speaking. – alephzero Aug 15 '16 at 13:05