A power trio is a rock and roll band format having a lineup of guitar, bass and drums, leaving out the second guitar or keyboard that are used in other rock music quartets and quintets to fill out the sound with chords. While one or more band members typically sing, power trios emphasize instrumental performance and overall impact over vocals and lyrics.
The rise of the power trio in the 1960s was made possible in part by developments in amplifier technology that greatly enhanced the volume of the electric guitar and bass. Particularly, the popularization of the electric bass guitar defined the bottom end and filled in the gaps. Since the amplified bass could also now be louder, the rest of the band could also play at higher volumes, without fear of being unable to hear the bass. This allowed a three-person band to have the same sonic impact as a large band but left far more room for improvisation and creativity, unencumbered by the need for detailed arrangements. As with the organ trio, a 1960s-era soul jazz group centered on the amplified Hammond organ, a three-piece group could fill a large bar or club with a big sound for a much lower price than a large rock and roll band. A power trio, at least in its blues rock incarnation, is also generally held to have developed out of Chicago-style blues bands such as Muddy Waters' trio.
In addition to technology improvements the other impetus for the power trio was the virtuosity of guitarists such as Eric Clapton, Jimi Hendrix, and Rory Gallagher who could essentially cover both the rhythm guitar and lead guitar roles in a live performance. In 1964 Frank Zappa played guitar in a power trio The Muthers together with Paul Woods on bass and Les Papp on drums.  In 1966 the prototypical blues-rock power trio Cream  was formed, consisting of Eric Clapton on guitar/vocals, Jack Bruce on bass/vocals, and Ginger Baker on drums. Other influential 1960s-era blues rock / hard rock power trio bands were The Jimi Hendrix Experience,  Blue Cheer, Grand Funk Railroad,  The James Gang featuring Joe Walsh, and Taste.  Well-known 1970s-era power trios include the Canadian progressive rock groups Rush and Triumph (band), the American band ZZ Top,  the British heavy metal band Motörhead, and Robin Trower. The band Emerson Lake and Palmer while replacing the guitarist for a keyboardist is usually considered as a power trio   as Emerson fulfills the rhythm and lead playing on the keyboards that would fall on the guitarist, while the bassist Greg Lake is the vocalist.
After the 1970s the phrase "power trio" tended to be applied to just about any three-piece rock bands. These bands include New Wave groups like The Police,  grunge band Nirvana, post punk band Hüsker Dü, hard rock/progressive metal band King's X, post-grunge band Silverchair, alternative bands The Presidents of the United States of America, Primus and Eve 6, pop-punk bands such as Green Day, Blink-182, Alkaline Trio and MxPx, and Argentine rock bands like Soda Stereo and Divididos. Also, by the 1990s, rock trios began to form around different instrumentation, from the band Morphine featured a baritone saxophone instead of an electric guitar, while Ben Folds Five replaced the guitar with various keyboards, principally the piano.