The classic Marshall stack is a symbol of loud rock music. The stack normally consists of one head containing the actual amplifier on top of one or two quad boxes, which are loudspeaker enclosures each containing four 12 inch loudspeakers arranged in a square layout. The top (or only) quad box normally has the top two loudspeakers angled slightly upwards, giving the Marshall stack a distinctive appearance. When two quad boxes are used, one on top of the other with the head on top, the complete unit is called a double stack, and only the top box has these angled loudspeakers.
For even greater power, a second or even a third 'slave' head can be driven from the master amplifier, each additional amplifier head driving another two quad boxes. This was taken to its logical conclusion in the early 1970s by the band Blue Oyster Cult, who used an entire wall of double-stack Marshall amplifiers as their backdrop.
Vox and Fender were two of the main competitors in the 1950s.