askchapo

The prescriptions for when you're "supposed" to use one or the other are as follows:

• A hyphen is for joining words together or indicating that a word has been split in the middle and will continue on the next line.
• An en dash is for ranges of numbers, or at the start of each item in a list. The en dash is also identical, at least in the font I'm looking at, to the minus sign used in math, as well as to the figure dash used for phone numbers and metrical feet. These symbols have separate Unicode points.
• An em dash is for a lot of other things, most notably uses similar to a colon or parenthesis, or as a way to show interruption. The em dash is also identical, at least in the font I'm looking at, to the quotation dash, which obviously has a separate Unicode point because we need as many Unicode points dedicated to singular straight horizontal lines as possible.

There are in fact even more horizontal line symbols with Unicode points than even these six.

But I myself never use en dashes: Ranges in numbers get a ~ like in CJK languages; lists get a hyphen or some other symbol; minuses are also hyphens; phone numbers get hyphens or spaces; and I can count on one hand the number of times I've had to write down a metrical foot.