"I Got Rhythm" chord progression

I Got Rhythm (IGR)
32 bar form

“I Got Rhythm (IGR)" is composition by composer George and Ira Gershwin, originally written as a slow number for the 1928 musical “Treasure Girl”. A faster version was made famous in 1930 with “Girl Crazy”. Originally written in Db major. Its most popular key has become Bb major.

Not only has I Got Rhythm become a jazz standard, the chord progression of this piece has inspired the 32 bar form, which became the most popular music form written in the 20th century. It was used extensively in popular works written in the commercial song writing industry of New York City, called "Tin Pan Alley," and by BeBop composers in the 40s and 50s.

In Honor of IGR, jazz musicians call these chord changes “Rhythm Changes”. So that they could feature the IGR form in their work without having to pay royalties to the Gershwin Estate, they reduced the measures from 34 to 32 bars, changed the key, and made various chord substitutions. Many of Charlie Parker’s compositions were based on Rhythm Changes such as Moose the Mooche, Steeplechase and Anthropology.

There are many alterations and exceptions to these rules, but a typical Rhythm Changes tune will contain most if not all of these elements:

• 32-bar AABA (or more specifically A A’ B A’) form.
• Begins and ends in the tonic key.
• Moves to the IV chord in bars 5 and 6.
• Bridge based on a dominant cycle progression III-VI-II-V.

In the key of Bb, the progression would be as follows:

The "rhythm changes" is a thirty-two-bar AABA form containing four eight measure sections. In roman numeral shorthand, the first chords used in the first 4 measures of the "A" section are:

A:   | I vi | ii V | I vi | ii V |
       | move to | iv chord |   turn | around   |

It is played twice. Notice that the first four bars are stating the tonic key of Bb major, from which bars 5 and 6 move next to the IV key (Eb major), and  before using the last two bars as a "turnaround" back to the top of the form. Even after these chords are altered, those harmonic pillars will remain the same; they are characteristic of A: section rhythm changes chords. The function of the "turnaround" is to leave the music unresolved and calling for continuance, so the movement can lead back to the beginning of A in the first verse. The Second A: part is a progression called a "Cadence" which resolves in the Tonic I chord, and is designated part A': .    The concluding 4 measures will proceed to the "cadence" resolving in the Tonic I chord as follows. 

A':  | I vi | ii V | I vi | ii V |
       | I I7 | IV iv | I V | I |

The B: part, or Bridge section of rhythm changes (bars 17-24) uses one chord every 2 bars in the III7-VI7-II7-V7 progression.

B:   |    III7         |     III7     |       VI7    |     VI7     |

       |      II7        |      II7      |       V7     |      V7     |

The bridge

The "bridge" consists of a series of dominant sevenths that follow the circle of fourths (ragtime progression, which is circle of Fifths backwards), sustained for two bars each and thus conveying the sense of a shifting key center. It was such a stock feature of composers in Tin Pan Alley, that it became known as the "commercial", "Montgomery Ward" or "the Sears Roebuck" bridge.

Written in the Key of Bb, a IGR progression might look like this:

A:      | BbM7, Gm7| Cm7,    F7   | BbM7, Gm7 |  Cm7, F7 |

          | Fm7 Bb7 | EbM7, Ebm7| Dm7,    Gm7|   Cm7, F7 |

A':     | BbM7, G7 | Cm7,    F7   | BbM7, Gm7 |  Cm7, F7 |

          | Fm7 Bb7 | EbM7, Ebm7|   Cm7,  F7  |   BbM7      |

B:  |  D7 | D7  |  G7 | G7

|  C7 | C7 |  F7 |  F7

A':     | BbM7, Gm7| Cm7,    F7   | BbM7, G7 |  Cm7, F7 |

          | Fm7 Bb7 | EbM7, Ebm7|   Cm7,  F7  |   BbM7   |

You can think of the B: chords in two ways: one is the III-VI-II-V numerals mentioned already, and the second is that each chord is the V7 of the next chord in the progression, creating a descending extended  fifths or a secondary fifths progression. They therefore follow the keys in the Circle of Fifths backwards to the original key.  For example: D7 is the V7 of G, which is the V7 of C, which is the V7 of F, which is the V7 of Bb (root or tonic key); this takes you back to the opening A idea and "resolves" the chord.  This progression makes use of how the V degree (called the dominant) seems to resolve tension when it is followed by the I chord degree (called the root, or tonic) , giving the listener a sense the movement of the music has returned "home."  

Notice that the major III, II and IV degrees are not in the Bb scale. Dm (iii) and Gm (ii) are minor in Bb.  The concept of secondary fifths chord substitutions may be expressed in chord degrees with Roman Numerals as a fraction where the numerator is the degree of the denominator: so the V7/vi (read as the 5th of the 6th).

Observe how this works in the extended fifths. After A': ends on the tonic Bb (I), B: with D7 which is both the III, and the 5th degree or Dominant (V7) chord of the sixth degree the key of G or the (VI) chord of Bb (V7/VI); G (VI) which is the fifth (V7) degree of key of C or the (II) of Bb (V7/II); C which is the fifth (V7) of the key of F or the (V) of Bb (V7/V) , and finally F is the (V7) of the tonic (I) Bb in the key of Bb(V7/I ).

With this in mind, the B part may be written here as follows:

B | V7/VI | V7/VI | V7/II | V7/II |
| V7/V | V7/V | V7/I | V7/I |

The IGR chord progression can also be written in the 7 degree intervals of a major key using Roman Numerals: Capital to indicate Major Chords, lowercase to indicate minor chords.  Because Jazz uses chords with 4 notes or more, the "quality" of the chords includes degrees beyond a triad's 5th degree, as well as further qualities such as Major, minor, sharp, or flat.

Substitutions

The jazz standard chord progression of "rhythm changes" uses a simple eight bar chord progression in the bridge with the chords III7, VI7, II7, V7; in the key of B♭, these chords are:

B:   |  D7 | D7  |  G7 | G7
       |  C7 | C7   |  F7 |  F7

A jazz performer might add a "ii-V7 substitution" to each chord.  The ii-V substitution is when a chord or each chord in a progression is preceded by its supertonic (ii7) and dominant (V7), or simply its dominant. For example, a C major chord would be preceded by Dm7 and G7 (the standard ii-V7-I progression).

The above bridge would then be:

B: | Am, D7 | Dm, G7|
     |  Gm, C7 | Cm,F7 |

Since secondary dominant chords are often inserted between the chords of a progression rather than replacing one, this may be considered as 'addition' rather than 'substitution'.

Alternatively, tritone substitutions could be applied to the progression.  "Tritone Bridge" is a term used to describe using tritone substitutions throughout the bridge of a rhythm tune. Because each chord in the Bridge is a 7th chord, you can also add in a iim7 chord before any chord in this section, creating a series of ii-V’s over this 8-bar progression.

1. Rhythm Changes Bridge (in the key of Bb)
   B:|| D7      | D7          | G7      | G7      
       | C7       | C7          | F7      | F7  
2. Dominant chords converted to ii Vs
   B:   |  Am7  | D7   |  Dm7 | G7
          |  Gm7 | C7   |  Cm7 |  F7
3. Full tritone substitution for each ii V, i.e. the "Tritone Bridge"
   B:|| Am D7  | Ebm Ab7  | Dm G7  | Abm Db7
        | Gm C7  | Cm- F#7   | Cm F7  | F#m B7 

Notice that the original changes are all in bars 2, 4, 6 and 8, while their related iim7 chords all fall in bars 1, 3, 5 and 7 of the Bridge section.