Intonation refers to the ability of a guitar to keep it's tuning as it is played down the fretboard. For example, if playing the open E string yields a perfect E, playing an E at the 12th fret should also yield a perfect E. This is determined by the distance between the nut and the bridge of the guitar, as well as the condition of the strings and frets. Most electric guitars allow intonation to be adjusted, either by moving the whole bridge, or each individual string such as with a Gibson tune-o-matic bridge. Acoustic guitars, in general, cannot adjust intonation without altering the bridge in a somewhat permanent manner.