An acoustic guitar is a musical instrument in the guitar family, that projects a sound wave from its vibrating strings through the air. The original, general term for the stringed instrument was called a guitar, while the retronym 'acoustic guitar' was created to distinguish it from an electric guitar which relies on an electronic amplification system. The sound waves from the strings of an acoustic guitar resonate through the instrument's body, amplifying the sound. Typically, a guitar's body is a sound box, of which the top side serves as a sound board that enhances the vibration sounds of the strings. In standard tuning the guitar's six strings are tuned (low to high) E2 A2 D3 G3 B3 E4. Guitar strings may be plucked individually with a pick or fingertip, or strummed to play chords. Plucking a string causes it to vibrate at a fundamental pitch determined by the string's length, mass, and tension. (Overtones are also present, closely related to harmonics of the fundamental pitch.) The string causes the soundboard and the air enclosed by the sound box to vibrate. As these have their own resonances, they amplify some overtones more strongly than others, affecting the timbre of the resulting sound.