Passage: Duplex printing (printing on both sides of the paper) can halve paper costs and reduce filing volumes, albeit at a slower page-printing speed because of the longer paper path. Formerly only available on high-end printers, duplexers are now common on mid-range office printers, though not all printers can accommodate a duplexing unit.

Question: What was formerly available only to high-end printers?
duplexers