MC4 connectors are single-contact electrical connectors commonly used for connecting solar panels. MC4 stands for the manufacturer Stäubli (formerly Multi-Contact) and a 4 for the 4mm (diameter) contact pin pin. MC4s allow strings of panels to be easily constructed by pushing the connectors from adjacent panels together by hand, but require a tool to disconnect them to ensure they do not accidentally disconnect when the cables are pulled. The MC4 and compatible products are universal in the solar market today, equipping almost all solar panels produced since about 2011. Originally rated for 600 V, most versions since are rated to 1000 V, which allows longer strings to be created.