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-inch, 3.5-inch, 2.5-inch or 1.8-inch 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.