Page "Movable type" Paragraph 91

Other large compartments in the California Job Case held spacers, which are blocks of blank type used to separate words and fill out a line of type, such as em and en quads ( quadrats, or spaces.

A quadrat is a block of type whose face is lower than the printing letters so that it does not itself print .).

An em space was the width of a capital letter " M " – as wide as it was high – while an en space referred to a space half the width of its height ( usually the dimensions for a capital " N ").