The guitar is one of the most popular instruments used as accompaniment to vocal performances. It has a tone range of over three octaves, and focuses on either harmonic accompaniment or lead. Playing a song on guitar, especially when trying to tailor it to match a vocalist, will sometimes require the songwriter to transpose guitar notation into a different key. In music, a key is the scale around which the song is based, and all chords and notes fall within the corresponding scale. To transpose guitar chords correctly, the songwriter must know the guitar fretboard and the relationships between musical notes and keys.
Each of the twelve major musical scales has seven degrees, or intervals, plus the octave of the root note. The C major scale, for example, consists of the seven whole notes in ascending order from C through B, plus the octave of C. Meanwhile, the G scale consists of G, A, B, C, D, E, F# and the octave of G. When guitarists transpose guitar music, they calculate the corresponding scale intervals and transpose the notes and chords according to their intervals to the root. A lack of proper transposition leads to a dissonant song.
A song written in the key of A is based around the A major scale: A, B, C#, D, E, F#, G# and A. An I-IV-V chord progression in the key of A would involve A, D and E chords because A, D, and E are the first, fourth and fifth intervals of this scale. In the key of D, on the other hand, this same progression would include the chords D, G and A — the same intervals of the D major scale. Learning all of the scale intervals make it possible to transpose guitar songs between any two keys; once the guitarist transposes the notes, the guitar needs to be physically re-tuned to the correct key.
When a guitar player needs to play songs in a transposed key, a capo is the tool most frequently used to quick-tune strings upward. The capo fits over the strings at the desired fret and holds the strings at that tone. The tone depends on the fret, and the capo sets the open tone. For example, setting a capo at the seventh fret of a guitar and playing an E chord shifts the entire chord's tone up seven half-steps, making it a B chord.