These are typesetter's terms.
an n-dash is really a hyphen the width of a lower-case n in the font. an m-dash is really a hypehn with width of a lower-case m in the font.
an m-dash is always longer than an n-dash in inkprint.
I hope this helps.
Can anyone enlighten me on m-dashes and n-dashes? What is their purpose as opposed to an ordinary dash?
Thanks
Don