Stringed instruments produce sound when their strings are caused to vibrate. The strings can be plucked, bowed or struck, depending upon the instrument. Keyed instruments, such as the piano or harpsichord, are composed of a series of hidden strings that are struck by small hammers when the appropriate key is pressed, which results in a clear, ringing sound with bright overtones. Instruments such as the cello or violin can be bowed to produce sustained notes or plucked to produce shorter pizzicato notes, depending upon the piece. The guitar and similar instruments are generally plucked, either by a musician's fingers or by a pick. Stringed instruments are chordophones, which means they are generally capable of producing a full chord independently.
The sound produced by an unamplified stringed instrument is minimal and generally inaudible in concert or orchestral settings. To remedy this, different cultures have produced different methods of amplifying the sound. Among the earliest steps taken to provide more resonant instruments was the introduction of the hollow body, which allowed the notes to reverberate within the instrument itself. F-holes, which were holes in the shape of italicized "f"s cut into an instrument's body, followed. "Resonator" instruments, which were made out of metal instead of the traditional wood, further refined the process of amplification. Electronic amplifiers for stringed instruments were introduced in the 1950s and have allowed instruments a variety of stringed instruments to be heard clearly even in stadium settings, as well as opening up the possibility of tonal manipulation through pedals and other devices that connect between the instrument and the amplifier.
The pitch produced by a vibrating string can be altered by shortening or lengthening the amount of string made to vibrate. Instruments descended from the lute lay the strings flat across a fretboard, which allows a musician to alter the length of the string by holding down upon the string itself, creating higher-pitched notes the higher up along the fretboard the string is pressed. Pianos and similar instruments, such as the harp, offer a set number of strings ordered by length from longest to shortest, going from left to right along the keyboard. The thickness of a string also plays an important role in pitch, with thicker strings producing a deeper sound than thinner ones. Though many instruments including the guitar and violin feature strings ordered thickest-to-thinnest, others such as the mandolin have at least one string that interrupt the logical order.