A guitar (classical, acoustic or electric) with seven strings instead of six.

The Russian Guitar

The classical and acoustic seven string guitars originated in 19th century Russia. Its invention is attributed to Andreas Sichra who also wrote the method for the guitar and seventy-five compositions. It has been then proclaimed to be a Russian guitar and has been primarily played in Russia and later the Soviet Union. While it did not replace the regular six string guitar in Russia, many of the more "patriotic" musicians such as Simeon Aksenov and Yuri Vizbor chose to play it as it is a part of Russian herritage. The Russian version of the seven string guitar has been used by professionals due to its great flexibility, but has also been popular with amateurs due to the relative simplicity of some basic chords. In the standard tuning, the A major chord is simply a barre on the second fret, the B major is a barre on the fourth, C major on the fifth, D major on the seventh etc.

The Electric Guitar

The electric version of the seven string guitar was introduced in the late 1930s by George Van Eps . The guitar was basically a regular electric guitar with an additional bass (usually Low-A) string. This feature allows playing bass, chords and solos simultaneously. Van Eps referred to this style of playing as "lap piano". While still considered a rare novelty, seven string electric guitars are occasionally used by popular metal bands (such as Korn) and virtuoso guitarists (such as Steve Vai and John Petrucci). The eight-string also has an underground virtuoso following.

Jazz guitarists using the seven-string instrument include Bucky Pizzarelli, Howard Alden, Ron Eschete, Lenny Breau, and John Pizzarelli, who is the author of the Foxwoods Casino theme and is the son of jazz legend Bucky Pizzarelli.

For tuning the guitar, see http://www.personal.psu.edu/users/f/p/fpm108/glw/lessons/lesson2.htm