Augmented Sixth Chords

Augmented Sixth Chord

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  One way to emphasize a tone is to approach it by a half step, either from above or from below
  
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  It is a strong approach to the dominant by half steps above or below
  

Example: Key of g minor C#-D

+6 V

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  This method of approaching the dominant distinguishes a whole category of pre-dominant chords called augmented sixth chords
  
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  The characteristic elements of most augmented sixth chords:
  
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    The chord being approached is the V chord
    
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    The minor-mode 6^th (chromatically lowered if in a major key) appears in the bass
    
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    The #4^th is in an upper part
    
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  The interval of an +6 formed by these pitches is enharmonically equivalent to a m7, but the difference between the effect of the +6 and that of the m7 is easily detected by the ear
  
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  m7 tend to resolve downward while +6 resolves upward
  

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  The analytical symbol of an augmented 6 is +6 and not a roman +VI
  
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  The interval of the +6 usually resolves outward by half step, following the tendencies of the tones to lead to a dominant
  
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  Less commonly, the top pitch of the +6 may descend chromatically to produce the 7^th of a V⁷. This occurs only when: i) +6 chords have three or more pitch classes; ii) the top pitch of the +6 (#4) interval in an inner part
  
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  +6 chord is among the strongest of all predominant chords, and it generally moves directly to V (or i -V)
  
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  Frequently used just after a modulation to make it clear to the listener that a modulation has, in fact, occurred
  
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  Like N⁶, the +6 originated in the minor mode, but it was soon found to be equally useful in major keys
  
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  When used in major keys, it is often preceded by mode mixture
  

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  In most cases, +6 chords contain more than two pitch classes
  
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  The third pitch class is usually the tonic pitch, which lies a M3 above the bass pitch (b6th)
  
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  This combination of notes is referred to as an Italian augmented sixth chord (It⁺⁶)