The classic Marshall stack consists of a Marshall amplifier head with at least 50 watts of power and two stacked 4 X 12" speaker cabinets, the top one typically a slanted cabinet. When used with a head with valves in the pre and power stages this type of amplifier setup provides a sound that has been a staple of rock and roll since the 60s. A head with only one such cabinet is called a half stack. These days most brands of guitar amplifier are reffered to as such when in a similar configuration: Laney stack, ENGL stack etc.