The most common number of strings on a guitar is six. This number is simply the result of the natural evolution in the instrument, which has undergone an unknowable number of transformations over the past 5000 years or so. Of course, while six may be the most common number of strings on the modern guitar, it is hardly the only number. Guitars of the 21st century are available in an almost inexhaustible supply of variations, including eight-string, ten-string, twelve-string and, as will be discussed herein, seven-string instruments.

The seven-string guitar has been used for at least 150 years in various cultures, such as Russian (more on this later). This history is evinced by the dated surviving works written for the instrument, such as those by Napoleon Coste, a French guitarist of the 19th century. The extra string of this instrument (or strings, depending on what number you start from) is intended to extend the bass range. On the modern six-string guitar, the seventh is usually added in one of two ways. The first and most common method is to increase the width of the fingerboard so that the additional string may be stopped by the left hand. The second method is to place the additional string so that it lies next to the existing bass strings, free of the fingerboard. This technique is similar to that used in stringing the archlute and theorbro. In the second method, the seventh string is obviously without frets.

As mentioned, Russian culture can lay claim to a traditional seven-string guitar. This instrument arrived in that country in the very early 19th century and was probably a development of another similar instrument. The invention of the seven-string Russian guitar is often attributed to Andrei Sychra, a prominent guitarist, composer and teacher of the late 18th and early 19th centuries, although this origin is sometimes disputed. What is certain is that Sychra wrote both method and compositions for the instrument, many of which still survive today. Russian seven-string guitars are lauded for their musical flexibility and sound. Its simple basic chords and easy-to-play alternating bass lines make it popular with amateurs as well as professionals. It is traditionally played with the fingers rather than a plectrum.

The seven-string electric guitar was originally developed in the United States in the late 1930s. It was used by many jazz guitarists of the era. The first bodies of the first instruments were hollow or semi-hollow, which created the distinctive sound associated with the jazz genre. It was not until the 1990s that solid-body seven-string guitars were commercially produced. During this decade, a number of artists were introduced to the instrument's extended range. Although it never became as popular as its six-string cousin, the seven-string guitar established itself enough to become a permanent fixture in the music industry.

Today, the seven-string guitar is a niche instrument, used primarily by progressive metal and rock guitarists. Its appeal continues to be its extended bass range, which allows for high melodies and solos played over deep riffs.