The Russian Tango class submarines (Project 641B Som [Catfish]) were the successors to the Foxtrot class submarine based in the Black Sea and Northern Fleet areas. The first of the class was completed in 1972 at Gorky. A total of 18 were built in two slightly different versions. The later type was several metres longer than the first possibly because of installation of ASW missile equipment. Coupled with new armament and sensor fit, the Tango class were ideal for ambush operations against Western nuclear submarines at natural chokepoints.