There use to be two types of similar connectors for keyboards, the AT and PS/2 connectors. Above you see a PS/2 6-pin connector on the left, and a 5-pin AT on the right. Physically they are similar but they are electrically different as the pinouts are different, the pins are also different in diameter and the shell casing is also a slightly different diameter. These connectors are also known as 6-pin mini-dins in the case of the PS/2 and 5-pin dins in the case of the AT connector. Today, the PS/2-style can still be found in use, but it is quickly being replaced by USB-attached keyboards.