Half-height drive bays are 1.625″ high by 5.25″ wide (), and are the standard housing for CD and DVD drives in modern computers, but were sometimes used for other things in the past, including hard disk drives (roughly between 10 and 100 MB) and floppy disk drives.
More recently, it is becoming common to use 3.5″ bays for smart card and memory card readers, or for panels with additional USB ports.