This page covers those mains plugs and sockets which are not in common domestic use but do not count as industrial or multiphase either.
Old Spanish sockets
Older buildings in Spain may have sockets that take a particular type of plug that has two flat contacts and a round earth pin, somewhat similar to the ones found on American plugs.
The live and neutral measure 9 mm × 2 mm × 19 mm, and are 30 mm apart. The earth pin is a cylinder with a diameter of 4.8 mm, and is 19 mm long.
While the plug may look like a USA-style connector at first glance, the spacing between the two flat contacts on this obsolete plug is much wider than found on a standard American plug. Therefore, American plugs will not fit in these sockets.
No appliances are sold with these plugs. Adaptors are necessary.
The British electric clock connector
Fused plugs and sockets of various proprietary (and non-interchangeable) types are often seen in older public buildings in the UK where they are used to feed AC electric wall clocks. They are physically smaller than conventional socket outlets, commonly being made to fit BESA junction boxes, and often of very low profile, for neatness. Early types were available fused in both poles, later types fused in the live only and provided with an earth pin. Most are equipped with a retaining screw or clip to prevent accidental disconnection. The prevalence of battery powered quartz controlled wall clocks has meant that this connector is rarely seen in new installations.
NEMA 2-15, and 2-20
These ungrounded (unearthed) plugs with two flat parallel pins are variants of the 1-15 but are intended to deliver 240 volts instead of 120. The 2-15 has both pins rotated 90 degrees from vertical and is used for 240 V service at 15 amperes, while the 2-20 has one pin rotated 90 degrees and is used for 240 V service at 20 amperes. NEMA 2 plugs and sockets are rare because they have been prohibited for household use in the United States and Canada for several decades. They are potentially hazardous since they have no ground or neutral, and in some cases plugs can be inserted into incorrect-voltage sockets. Prior to the adoption of the NEMA standard, a plug nearly identical to the 2-20 was used for 120 V at 20 A. A 2-20 plug would fit into either a 5-20 or a 6-20 socket (explained below) which use different voltages.