The cards added to and extended the functionality of the base motherboard when paired with specialized software that enabled the computer to read the input/output of the devices on the other side of the cable (the peripheral) or to take advantage of chips on the board - as was the case with memory expansion cards.
These slots could hold printed circuit board cards with double-sided edge connectors, 25 "fingers" on each side, with 100 mil (0.1 inch) spacing between centers.
Slot 3 in an Apple IIe that has an 80-column card fitted (which is usually the case) and Slots 1 through 6 in a normally configured Apple IIgs are "virtually" filled with on-board devices which means that the physical slots cannot be used at all, or only with certain specific cards, unless the conflicting "virtual" device is disabled.
In addition to the seven standard expansion slots, the following computers contained additional, largely special-purpose expansion slots: Perhaps the most common cards found on early Apple II systems were the Disk II Controller Card, which allowed users of earlier Apple IIs to use the Apple Disk II, a 5¼ inch, 140 kB floppy disk drive; and the Apple 16K Language Card, which increased the base memory of late-model Apple II and standard Apple II Plus units from 48 kB to 64 kB.
[citation needed] Apple II cards can be broadly divided into the following categories: