Sentence:  Horizontal spacing is further divided into multiples and fractions of the em. The multiples are called "quads". The fractions are called <WSD>"spaces</WSD>". 
Meaning of the word marked with <WSD> tags:  A piece of metal type used to separate words, cast lower than other type so as not to take ink, especially one that is narrower than one en (compare "quad"). (from 17thc.)