example, from C's vi–ii–iii progression Am–Dm–Em, the chord Em is often played as E or E7 in a minor chord progression. Among basic chords, the minor chords (D,E,A) are the tonic chords of the relative minors of the three major keys (F,G,C): The technique of changing among relative keys (pairs of relative majors and relative minors) is a form of modulation. Minor chords are constructed by the harmonization of minor scales in triads. Seventh chords: major–minor chords with dominant function Adding a minor seventh to a major triad creates a dominant seventh (denoted V7). In music theory, the "dominant seventh" described here is called a major-minor seventh, emphasizing the chord's construction rather than its usual function. Dominant sevenths are often the dominant chords in three-chord progressions, in which they increase the tension with the tonic "already inherent in the dominant triad". The dominant seventh discussed is the most commonly played seventh chord. An A-major I–IV–V7 chord progression A–D–E7 was used by Paul McCartney in the song "3 Legs" on his album Ram. These progressions with seventh chords arise in the harmonization of major scales in seventh chords. Twelve-bar blues Be they in major key or minor key, such I–IV–V chord progressions are extended over twelve bars in popular music—especially in jazz, blues, and rock music. For example, a twelve-bar blues progression of chords in the key of E has three sets of four bars: E–E–E–E7 A–A–E–E B7–A–E–B7; this progression is simplified by playing the sevenths as major chords. The twelve-bar blues structure is used by McCartney's "3 Legs", which was noted earlier. Playing chords: open strings, inversion, and note doubling The implementation of musical chords on guitars depends on the tuning. Since standard tuning is most commonly used, expositions of guitar chords emphasize the implementation of musical chords on guitars with standard tuning. The implementation of chords using particular tunings is a defining part of the literature on guitar chords, which is omitted in the abstract musical-theory of chords for all instruments. For example, in the guitar (like other stringed instruments but unlike the piano), open-string notes are not fretted and so require less hand-motion. Thus chords that contain open notes are more easily played and hence more frequently played in popular music, such as folk music. Many of the most popular tunings—standard tuning, open tunings, and new standard tuning—are rich in the open notes used by popular chords. Open