Design work on the AK-47 began in 1945. It was presented for official military trials the following year, and in 1948 the fixed-stock version was introduced into active service with selected units of the Soviet Army. An early development of the design was the AK"S" ("Skladnoy", or 'folding'), which was equipped with an underfolding metal shoulder stock. In early 1949, the AK was officially accepted by the Soviet Armed Forces and used by the majority of the member states of the Warsaw Pact.