Hyphens, En Dashes, Em Dashes

Q. What is an en dash?

A. An en dash is a useful but subtle creature that is longer than a hyphen but shorter than an em dash. (An “em” is a typesetting measure of width equal to the size of the type. For instance, in eight-point type, an em is eight points wide. An “en” is half an em.) Our main uses for the en dash are in inclusive numbers (129–35) and in adjectival phrases that contain an open compound (the post–Civil War period).

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]