The clip-on connectors are also known as EM connectors. There are two types of plugs. It is therefore important to indicate when purchasing connectors whether a closed or an open connection type is required. A closed-end connector is used when the air is supplied directly from a main air line (ie, a compressor or air receiver). A check valve loosens when it is connected to the tire’s valve, allowing air to flow into the tire. An open-end connection is used with the MK3 tire inflator, PCL wall pressure gauges, and PCL handheld pressure gauges. This connector has a pin that lifts the tire’s valve when in use. The plugs are available with both a closed and an open valve.