"Schuko" () is a registered trademark referring to a system of AC power plugs and sockets that is defined as "CEE 7/3" (sockets) and "CEE 7/4" (plugs). A Schuko plug features two round pins of 4.8 mm diameter (19 mm long, centers 19 mm apart) for the line and neutral contacts, plus two flat contact areas on the top and bottom side of the plug for protective earth (ground). The socket (which is often, in error, also referred to as CEE 7/4) has a predominantly circular recess which is 17.5 mm deep with two symmetrical round apertures and two earthing clips on the sides of the socket positioned to ensure that the earth is always engaged before live pin contact is made. Schuko plugs and sockets are symmetric AC connectors. They can be mated in two ways, therefore line can be connected to either pin of the appliance plug. As with most types of European sockets, Schuko sockets can accept Europlugs. Schuko plugs are considered a very safe design when used with Schuko sockets, but they can also mate with other sockets to give an unsafe result.