The plugs have three round pins arranged in a triangle, with an earthing pin. The plugs are polarized and unfused. Plugs are non-interchangeable between current ratings. Introduced in 1934, the BS 546 type has mostly been displaced in the UK by plugs and sockets to the BS 1363 standard. According to the IEC some 40 countries use Type D and 15 countries use Type M. Some, such as India and South Africa, use standards based on BS 546.