Strictly speaking, the en dash is the width of an N, an em the width of a M
(capital letters). In general terms this makes an en dash about twice as
long as a hyphen in a typical font, an em dash three times as long. The
distinction of en and em dash uses varies between U.S. and U.K. usage, as I
learned to my chagrin after a long exchange with a user who I thought was
trying to make an en dash do something it wasn't meant to do!
