how did the em dash get its name
Dash The dash is a punctuation mark that is similar in appearance to a hyphen or minus sign, but differs from both of these symbols primarily in length and function. The most common versions of the dash are the en dash (–) and the longer em dash (—), whose names historically were loosely related with the length of a lower-case n and upper-case M, respectively, in commonly used typefaces.
Em (typography) In Cascading Style Sheets, the em unit is the height of the font in nominal points or inches. The actual, physical height of any given portion of the font depends on the user-defined DPI setting, current element font-size, and the particular font being used.