Full-height

A drive bay is a standard-sized area for adding hardware to a computer. Most drive bays are fixed to the inside of a case, but some can be removed.

Over the years since the introduction of the IBM PC, it and its compatibles have had many form factors of drive bays. Four form factors are in common use today, the 5.25", 3.5", 2.5" or 1.8" drive bays. These names do not refer to the width of the bay itself, but rather to the width of the disks used by the drives mounted in these bays.

8.0″ drive bays were found in early IBM computers, CP/M computers, and the TRS-80 Model II. They were 4.624″ high by 9.5″ wide, and approximately 14.25″ deep (117.4 mm × 241.3 mm × 361.9 mm), used for hard disk drives and floppy disk drives. This form factor is obsolete.

Full-height bays were found in old PCs in the early to mid-1980s. They were 3.25″ high by 5.75″ wide, and up to 8″ deep (82.55 mm × 147.64 mm × 203.20 mm), used mainly for hard disk drives and floppy disk drives. This is the size of the internal (screwed) part of the bay, as the front side is actually 5.875″ (an extra eighth). The difference between those widths and the name of the bay size is because it is named after the size of floppy that would fit in those drives, a 5.25″-wide square.

Half-height drive bays are 1.625″ high by 5.25″ wide (41.3 mm × 146.1 mm), 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. As the name indicates, two half-height devices can fit in one full-height bay. Often represented as 5.25-inch, these floppy disk drives are obsolete.

The dimensions of a 5.25″ floppy drive are specified in the SFF standard specifications SFF-8500 and SFF-8501 which were incorporated into the EIA-741 specification by the Electronic Industries Association (EIA). Dimensions of 5.25 optical drives are specified in the SFF standard SFF-8551 (they are somewhat shorter and not only size of the body is standardized, but also size of the bezel).

...
Wikipedia