Major 6th chord
In modern popular music, a sixth chord is any triad with an added sixth above the root as a chord factor. This was traditionally (and in classical music is still today) called an added sixth chord or triad with added sixth since Jean-Philippe Rameau (sixte ajoutée) in the 18th century. It is not common to designate chord inversions in popular music, so there is no need for a term designating the first inversion of a chord, and so the term sixth chord can be used in popular music as a short way of saying added sixth chord. When not otherwise specified, it usually means a major triad with an added major sixth interval (a major sixth chord). However, a minor triad is also used, together with the same interval, resulting in a minor sixth chord (also known as minor major sixth).
What in popular music is called a sixth chord was traditionally called an "added sixth chord". As the name suggests, this is a triad with an added sixth interval. It is generally built on the sub-dominant note (fourth scale degree), although it can be built on any note. Typically, the triad is a major triad and the additional sixth interval is major (major sixth chord). For example, a major sixth chord built on C (denoted by C6, or CM6) consists of the notes C, E, G, and the added major sixth A Play (help·info). These are the same notes as those of an A minor seventh chord - whether such a chord should be regarded as an added sixth chord or a seventh depends on its context and harmonic function. To explain the analyses as added sixth chords, against common practice period theory, provides the example of the final tonic chord of some popular music being traditionally analyzable as a "submediant six-five chord" (added sixth chords by popular terminology), or a first inversion seventh chord (possibly the dominant of the mediant V/iii). According to the interval strengths of the added sixth chord the root of the strongest interval of the chord in first inversion (CEGA), the perfect fifth (C-G), is the bottom (C), the tonic.
In jazz, the minor sixth chord (sometimes: minor major sixth, or minor/major sixth) Play (help·info) is frequently used. It is unlike the major sixth chord, which is often substituted for a major triad; the minor sixth acts as a tonic or subdominant in the minor mode. The chord consists of a minor triad with a tone added a major sixth above the root; thus in F♯, it would contain the notes F♯, A, C♯, and D♯. This chord might be notated F♯m6, F♯m/M6, F♯min/maj6, F♯min(maj6), etc. Note that F♯m6 has the same notes as B9 with the root omitted, i.e. the notes B, D♯, F♯, A, and C♯. These notes form a tetrad with several enharmonic equivalents: F♯/A/C♯/D♯ might be written as F♯m6, B9, B9 (no root), D♯m7♭5, F7♭9, or F7alt. Many jazz chord charts use these chord notations indiscriminately, particularly in the choice of minor sixth versus dominant ninth chords. Thus, in some cases when a F♯m6 is indicated, the B9 is in fact a better harmonic choice, i.e. closer to the composer's harmonic intent; or vice versa. Analysis of the movement of the root, in the presence of dominant-functioning harmonies, will generally indicate which enharmonic chord is the appropriate notation choice. In some cases, the harmony is ambiguous. The notes are those of the half-diminished seventh chord: for example B D F♯ G♯ being both the G♯ half-diminished seventh / G♯m7(♭5), and Bm6.
Examples that use the major 6th chord include the opening of the chorus of "Skylark" by Johnny Mercer and Hoagy Carmichael, and "Moonlight in Vermont" by John Blackburn and Karl Suessdorf. In both examples the melody begins the chorus on the major 6th.
Sixth, sixth chord, and added sixth
In music, the sixth factor of a chord is the note or pitch six scale degrees above the root or tonal center (see sixth chord). When the sixth is the bass note, or lowest note, of the expressed chord, the chord is in third inversion Play (help·info). However, this is equivalent to a seventh chord.