The Who, formed in London in 1964, is a British rock group among the most popular and influential bands of the 1960s and ’70s. Their contributions to rock music include the development of Marshall Stack, large PA systems, the use of the synthesizer, Townshend’s feedback and power chord guitar technique, and the development of the rock opera. They are cited as an influence by many hard rock, punk rock, power pop, and mod bands, and their songs are still regularly played.