In hot metal typesetting, this occurs when a typeface is cast on a body block that is either larger or smaller than its standard point size. A ten-point face cast on a twelve-point body creates built-in leading, saving the compositor from inserting separate spacing strips between lines. It allows for much faster setting of text that requires generous line spacing.
Heard on the job
We're setting the main body copy in a b****** type so we don't have to hand-lead every single line.
Insider tip
Using this type of casting was a massive time-saver for large volumes of text, but it meant the shop had to buy and store an entirely separate set of matrices or cast type.