If you're a musician or you've ever been around musicians, you have probably heard the terms "a blues", "bridge", and "AABA" used in casual conversation. Maybe you've wondered what these terms mean - this is a detailed explanation of what they mean, along with some information about what makes up a song.
Any jazz tune - in fact, any tune in western music - is divided into measures when it is notated in sheet music form. Measures divide short sections of a tune into equal-length categories to make sheet music easier to read. A measure is also known as a bar.
Most tunes in jazz consist of 12, 16, 24, 32, or 64 measures; however, some tunes consist of different numbers of measures: for example, the Brazilian jazz tune "Girl from Ipanema" is 40 measures long, and the jazz tune "Invitation" is 48 measures in length.
But why not more straight-forward numbers, like 10, 50, or 100? While this is largely to do with traditions in Western music, the best explanation for using numbers like 12 and 32 is that they are multiples of four. Think of those numbers mentioned in the previous paragraph (12, 16, 24, 32, 40, 48, and 64) and compare them with these four-related multiplication facts: 4 x 3 = 12, 4 x 4 = 16, 4 x 6 = 24, 4 x 8 = 32, 4 x 10 = 40, 4 x 12 = 48, and 4 x 16 = 64.
Another number that can often be multiplied with another number to get the number of measures in a tune is 8. For example: 8 x 2 = 16, 8 x 3 = 24, 8 x 4 = 32, 8 x 5 = 40, 8 x 6 = 48, and 8 x 8 = 64.
Coming back to the point of this article, while all of these multiplication facts are very interesting, they do not tell the whole story about song structures: we can actually more accurately explain song structures by dividing songs in two categories based on structure*: "short", non-repetitive tunes, and "long" tunes that are more repetitive. There are exceptions, and a short tune played slowly enough can be longer than a "long" tune played quickly, but most of the time tunes will be either short and non-repetitive, or long and repetitive.
The "short" tunes are usually 12 or 16 bars in jazz. An example is "Watermelon Man", a tune written by Herbie Hancock that is 16 measures/bars in length and does not have repeating sections unless you play it multiple times.
A non-jazz example of a "short" tune is "Happy Birthday", which is only 8 bars long. While the Happy Birthday song is more than a little repetitive, it does not have whole repetitive sections that are longer than just a short phrase or two. Also, sometimes musicians who are not too interested in composing will write a short tune that has no repeating sections but consists of a few repeated phrases - for example, "Sonnymoon for Two".
The "long" tunes are usually 24, 32, 40, 48, or 64 measures, and they are repetitive or contain phrases repeated in a different tonality - for example, "All the Things You Are" and "Satin Doll". But there's a catch - many tunes that are only 12 measures long, when they're converted to a waltz, are 24 measures in length. Those tunes are 24 measures but they're really "short" tunes, because they're not very long and don't have repeating sections. Last but not least, many "short" tunes are played twice to make them longer - therefore they contain repeated sections and become "long" tunes. An example of a tune that can be made longer by playing it twice is "Ladybird".
So why does all this defining tunes as "long" and "short" matter? A simple explanation is that, by dividing songs into categories based on length, it is easier to understand what terms like "a blues" or "AABA" mean. By calling all tunes either "long" or "short", we can understand how many similarities there are between jazz tunes, and in the process easily connect musical terms for tune formats with either "long" or "short" tunes. It also provides a lot of background information and context that will make terms like "AABA" easier to understand.
Tunes referred to as a blues are always 12 measures (unless they're played twice) in length and are therefore "short" tunes. Also, every blues has the following chord sequence, or at least something like it:
(By the way, the vertical lines in this diagram separate the measures.)
| C7 | F7 | C7 | C7 |
| F7 | F minor | C7 | A7 |
| Dmin7 | G7 | C7 | C7 |
An AABA-format tune is usually 32 measures in length and therefore would be "long". AABA-format tunes are also very repetitive: they consist of two 8-measure sections that are identical or almost identical, a third 8-measure section that is different (known as the "bridge"), and then finally another 8-measure section like the first two. In other words, if the letter "A" is used to represent the three similar sections of the song and "B" is used to represent the different bridge section, the pattern of song sections is "AABA".
There are also some 64-measure tunes that use the AABA format. These tunes include four 16-measure sections instead of four 8-measure sections.
So, to summarize this information, the term "blues" refers to a particular chord sequence that is usually 12 measures in length, "AABA" refers to a tune with a total of three similar sections at the beginning and end along with a different section near the middle of the tune, and the term "bridge" refers to the "B" section of an "AABA"-format tune. Also, the number of measures in a tune is usually a multiple of 4.
*The terms "long" tunes and "short" tunes are not technical terms that would be used by musicians; we have coined these terms in an attempt to classify tunes by length.