When we talk about different sections in a piece of music we often make use of simple designations like "A" and "B". For example, if a piece begins with one idea and presents it in several phrases ending with a cadence or some sort of cadence equivalent, then we would label this whole section "A". Then if the piece goes on to present new and contrasting material in another independent section, we would label that "B". A very common kind of musical form then presents the A again so the form of the whole piece would be labeled ABA. There are thousands of medium-length pieces in ABA form which is also called "ternary" form.
One of the most common kinds of ternary forms is the minuet and trio, which is often used as the third movement in a host of string quartets and symphonies. Here is the third movement from Haydn's String Quartet op. 20, no. 4:
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Let's have a listen to the movement. This is the Attacca Quartet:
The whole thing is only two minutes long, so why am I using it as an example of "medium length" pieces? The answer is that this is a really convenient way to introduce the idea of ternary form and it is easy to hear in this example. Most minuets and trios, in exactly this form, are longer. I am thinking of shorter pieces as being up to around four or five minutes and medium length ones as being from around five minutes to perhaps ten or twelve minutes in length.
Go listen to the movement again and this time try and follow it in the score. The first thing to note is the repeat marks. In the second set of staves you will see that the second measure is incomplete and has a big double bar with two dots on each staff. This indicates that you go back and repeat the whole section, in this case, from the pickup note in the first violin. The second time you go on to the second section, which is also repeated. Incidentally this whole first section, with two halves, each repeated, is the basic binary dance form we find in a gazillion Baroque dance pieces. Jean-Baptiste Lully (1632 - 1687) is credited with adding a contrasting section (called a "trio" because it was commonly scored for two oboes and a bassoon). The first section is then followed by the trio which is also in two sections, each repeated as you can see from the double bar and the two dots. Each of these two sections is AABB, but the whole is labeled ABA or ternary form. It is extremely easy to hear in this example because the trio is distinguished by being a solo for the cello. As soon as you hear the cello come forward, you are in the trio section. The letters at the end, M. D. C. mean "minuet da capo" or "play the minuet from the beginning."
I think I will save other examples for the next post.