Answer: muts

AnswerCrossword Clue
mutshaving the area of an em quad or the length of an em dash
mutsa. the square of any size of type used as the unit of measurement for matter printed in that type size b. (originally) the portion of a line of type occupied by the letter M in type of the same size
mutsMUT, a mongrel dog, also MUTT
mutsunit of measurement equal to about one-sixth of an inch (4 mm)
mutsthe letter M, m