The guitar is a plucked stringed musical instrument that probably originated in Spain early in the 16th century, deriving from the guitarra latina, a late-medieval instrument with a waisted body and four strings. The early guitar was narrower and deeper than the modern guitar, with a less pronounced waist.

From the 16th to the 19th century several changes occurred in the instrument. A fifth course of strings was added before 1600; by the late 18th century a sixth course was added. Before 1800 the double courses were replaced by single strings tuned E-A-D-G-B-E, still the standard tuning.

The 19th-century innovations were largely the work of Antonio Torres. The instrument that resulted was the classical guitar, which is strung with three gut and three metal-spun silk strings. Nylon or other plastic was later used in place of gut.

Among variant forms of the guitar are the 12-stringed, or double-course, guitar, and the Mexican jarana and the South American charango, both small five-course guitars. Lyre-shaped guitars were fashionable in 19th-century drawing rooms. Other forms of the guitar include the metal-strung guitar played with a plectrum in folk and popular music; the cello guitar, with a violin-type bridge and tailpiece; the Hawaiian, or steel, guitar, in which the strings are stopped by the pressure of a metal bar, producing a sweet, gliding tone; and the electric guitar, in which the tone depends not on body resonance but on electronic amplification.

The guitar grew in popularity during the 17th century as the lute and vihuela declined. It remained an amateur's instrument from the 17th to early 19th century. A few virtuoso guitarists, however, became known in Europe, among them Gaspar Sanz (fl. 1674), Robert de VisŽe (c. 1650-1725), Fernando Sor (1778-1839), and Joseph Kaspar Mertz (1806-56). Modern classical-guitar technique owes much to the Spaniard Francisco Tirrega (1852-1909), whose transcriptions of works by Bach, Mozart, and other composers formed the basis of the concert repertory.

In the 20th century, Andres Segovia gave the guitar further prominence as a concert instrument, and composers such as Heitor Villa-Lobos and Manuel de Falla wrote serious works for it; others (e.g., Pierre Boulez) scored for the guitar in chamber ensembles.

The guitar is widely played in the folk and popular music of many countries. In jazz ensembles it is part of the rhythm section and is occasionally played as a solo instrument. In popular music the guitar is usually amplified, and ensembles frequently include more than one instrument, a "lead" guitar for solos, another for rhythm, and a "bass" guitar to play bass lines.