only a subset of keys. "Sonny Landreth, Keith Richards and other open-G masters often lower the second string slightly so the major third is in tune with the overtone series. This adjustment dials out the dissonance, and makes those big one-finger major-chords come alive." Repetitive open-tunings are used for two non-Spanish classical-guitars. For the English guitar the open chord is C major (C–E–G–C–E–G); for the Russian guitar which has seven strings, G major (G–B–D–G–B–D–G). Mixing a perfect fourth and a minor third along with a major third, these tunings are on-average major-thirds regular-tunings. While on-average major-thirds tunings are conventional open tunings, properly major-thirds tunings are unconventional open-tunings, because they have augmented triads as their open chords. Regular tunings Guitar chords are dramatically simplified by the class of alternative tunings called regular tunings. In each regular tuning, the musical intervals are the same for each pair of consecutive strings. Regular tunings include major-thirds (M3), all-fourths, augmented-fourths, and all-fifths tunings. For each regular tuning, chord patterns may be diagonally shifted down the fretboard, a property that simplifies beginners' learning of chords and that simplifies advanced players' improvisation. The diagonal shifting of a C major chord in M3 tuning appears in a diagram. Further simplifications occur for the regular tunings that are repetitive, that is, which repeat their strings. For example, the E–G–c–e–g–c' M3 tuning repeats its octave after every two strings. Such repetition further simplifies the learning of chords and improvisation; This repetition results in two copies of the three open-strings' notes, each in a different octave. Similarly, the B–F–B–F–B–F augmented-fourths tuning repeats itself after one string. A chord is inverted when the bass note is not the root note. Chord inversion is especially simple in M3 tuning. Chords are inverted simply by raising one or two notes by three strings; each raised note is played with the same finger as the original note. Inverted major and minor chords can be played on two frets in M3 tuning. In standard tuning, the shape of inversions depends on the involvement of the irregular major third, and can involve four frets. It is a challenge to adapt conventional guitar chords to new standard tuning, which is based on all-fifths tuning. Intermediate chords After major and minor triads are learned, intermediate guitarists play seventh chords. Tertian harmonization Stacking of third intervals The fundamental guitar-chords—major and minor triads and dominant sevenths—are tertian chords, which concatenate third