(a standard tuned guitar)
The baritone guitar is a guitar with a longer scale length, typically a larger body, and heavier internal bracing, so it can be tuned to a lower pitch. Gretsch, Fender, Gibson, Ibanez, ESP Guitars, PRS Guitars, Music Man, Danelectro, Schecter, Jerry Jones Guitars, Burns London and many other companies have produced electric baritone guitars since the 1960s, although always in small numbers due to low popularity.Tacoma, Santa Cruz, Taylor, Martin, Alvarez Guitars and others have made acoustic baritone guitars.
The baritone guitar first appeared in classical music. The Danelectro Company was the first to introduce an electric baritone guitar in the late 1950s, and the instrument began to appear in surf music and background music for many movie soundtracks, especially spaghetti westerns. More recently, the baritone guitar has appeared in rock, metal and improvised music. With appropriate strings, some baritone guitars can play in the bass guitar range.
A standard guitar's standard tuning (from lowest-pitched string to highest) is E2-A2-D3-G3-B3-E4. While no standard tuning has been established for baritone guitars, popular tunings for the instrument are: a perfect fourth lower than a standard guitar (B1-E2-A2-D3-F?3-B3), a perfect fifth lower (A1-D2-G2-C3-E3-A3), or a major third lower (C2-F2-A?2-D?3-G3-C4). Typically strung with 13 gauge (.013-.062) or 14 gauge (.014-.068) baritone guitar strings, as well as 12 gauge (.012-.060) guitar strings can also be used.
Baritone acoustic guitars typically have larger bodies than standard guitars, and have longer scale lengths so the strings can be tuned lower while remaining at normal tension. On a standard guitar, the scale length (the distance from the nut to the saddle on the bridge) is typically 24.6 to 25.75 inches (625 to 654 mm). The most common scale lengths on a baritone range from 27 to 30.5 inches (690 to 770 mm). 
The Danelectro baritone was used by guitarist Duane Eddy in numerous recordings, including "Bonnie Came Back", "Because They're Young", "Kommotion", "My Blue Heaven", "Deep in the Heart of Texas", and "The Son of Rebel Rouser". The instrument was used almost exclusively on his best-selling 1960 album "The Twang's the Thang" and appears regularly on singles and albums throughout his career. The "twangy" sound of his guitars (which include Duane Eddy custom-builts by Guild, Gretsch and Gibson) augmented the even deeper twangy sound made by the Danelectro baritone. Eddy used the familiar black model and an unusual gray "Longhorn" model.
Singer Jimmie Rodgers also favored the baritone guitar, which can be heard in the opening bars of his recording of "Woman from Liberia" (1960).
Singer Glen Campbell used a baritone electric guitar on several of his big hit songs, most notably "Wichita Lineman" and "Galveston", where he played a distinctive baritone solo following the melody in both songs.
Baritone guitars became popular in heavy metal music during the late 1980s, as it became increasingly popular to employ lower guitar tunings and dropped tunings. Early examples include Carcass (using B standard) and Bolt Thrower (using A standard on Realm of Chaos).
Rock guitarists also use down-tuned guitars. Benjamin Burnley, the guitarist/singer from Breaking Benjamin, uses custom- built PRS and ESP baritone guitars for their songs in Drop A# tuning. Ko Melina of The Dirtbombs plays a Fender Jaguar Baritone Custom. Teppei Teranishi of Thrice plays a baritone on the "Fire" disc of The Alchemy Index and Major/Minor. Ian Mackaye plays a baritone guitar when playing with his band The Evens. Aerosmith's Joe Perry plays a six-string Fender baritone bass tuned down to a G (which was later stolen) on "Back in the Saddle" on the 1976 Rocks album, and currently uses a Music Man Silo Baritone.
Mike Mushok of the band Staind has a signature model baritone guitar manufactured by PRS Guitars. Prior to his PRS signature model, Mushok had a signature baritone guitar produced by Ibanez called the MMM1, and had a custom built fanned-fret baritone made by Novax called the Expression.
Robert Smith of The Cure has made the baritone guitar a major component of his dark and atmospheric tone since 1989's Disintegration. Schecter guitars have produced a Robert Smith signature model baritone guitar.
Jazz guitarist Pat Metheny used baritone guitars made by Linda Manzer on his 2003 solo album One Quiet Night and his 2011 solo album What's It All About. Ani DiFranco often plays a baritone guitar, including those by Alvarez, frequently employing alternate tunings. Clifton Hyde has had his acoustic baritone guitar featured in the music of Sigur Rós, Gato Loco, and Pape Armond Boye. Bob Lanzetti, guitarist for the modern fusion band Snarky Puppy, frequently employs an electric baritone guitar as well.
Numerous fingerstyle guitarists use baritone guitars, including Andy McKee, Don Ross, Martin Simpson, Sergio Altamura, Iain Micah Weigert, and Dave Amato. Don Ross plays a baritone by Canadian Luthier Mark Beneteau, and Simpson has played baritones made by English luthier Ralph Bown. Andy McKee plays a baritone guitar made by another Canadian Luthier Michael Greenfield. Brian Setzer played the Gretsch/TV Jones Spectra-Sonic baritone on the song "Mystery Train" during the Brian Setzer Orchestra tour.
Blues band MonkeyJunk features a baritone guitar instead of a bass guitar.
Australian musician Stu Thomas plays a Barracuda baritone guitar by Burns London, tuned an octave lower than a regular guitar. He uses it as a bass when playing with Dave Graney & The mistLY, and as a "regular" guitar when he accompanies himself solo as The Stu Thomas Paradox.
Guitar Prodigy Sungha Jung plays original and cover instrumentals on Lakewood Acoustic Baritone Guitars.