Guitar, stringed musical instrument, related to the lute, played by plucking. Unlike the lute, it has a flat back and curve-waisted sides. The guitar evolved in Spain during the Middle Ages, apparently from Arabic instruments introduced by the Moors. The 5-string Spanish guitar evolved in the 1500s, becoming the Spanish national musical instrument. The modern guitar has 6 strings, generally made of metal or nylon. The guitar has become an important instrument in blues, jazz, and rock music.