ATX style connector
An ATX style connector is a replacement for the older P8 and P9 AT style connector. It is one of the largest connectors inside a computer. It connects a power supply to an ATX style motherboard. As shown in the picture, the 20-pin cable is a multi-color cable and may be labeled as P1.
With the introduction of ATX-2, this cable is now a 24-pin cable and no longer a 20-pin cable.
In the pictures, the ATX cable connector has a small clip on the top, which snaps and holds the cable in place. This cable is also keyed, meaning it only connects in one direction.
A power supply with a 24-pin connector can be used on a motherboard with a 20-pin connector by leaving the four additional pins disconnected. However, if you have a 24-pin connection on your motherboard all 24-pins need to be connected. If you are using a power supply that does not have a 24-pin connector, you need to purchase a new power supply.
When using a connector like that shown above, note the arrows pointing to each other. For the cable to be correctly inserted, the arrows must point to each other.