A disk drive in which the storage medium is permanently attached within the device. The drive may contain more than one disk — the so-called disk stack. The individual disks in the stack are called platters. Both sides are coated and the platters are mounted on a common spindle. Early fixed disk drives used disks with a diameter of 36 inches. The size, in inches, has progressively decreased in the sequence 14, 10.5, 9, 8, 5.25, 3.5, 2.5, 1.8, 1.3. Not all manufacturers used all of these sizes but currently 3.5 inch disk drives are used throughout the industry, with capacities of many gigabytes. Portable PCs currently use 2.5 inch disk drives.