The structure of most of the music on the A-level courses, and particularly that from before the modern period, falls into one of several categories. We'll look at them in turn, but it's worth noting right at the start that pieces of real music rarely fall perfectly into a structural model, particularly the more complex ones like Sonata and rondo forms.
Binary - AB The simplest structural model, and one you'll find a lot in Baroque music in particular. There's two kinds of binary form, simple binary and rounded binary. Simple binary more or less does what it says on the tin, that is you get one section with one theme in the tonic key, and then usually another theme that starts in the dominant and moves back to the tonic. Just as common, if no more so, is rounded binary.
People often confuse rounded binary for ternary form, because it appears to go back to the A material in the original key after the B section. The key thing to remember is that the first section in a rounded binary piece is tonally open, that is, it usually goes from the tonic to the dominant in a major key, and from the tonic to the relative major (III) or dominant major (V) when in a minor key. Ternary form is different because A is tonally closed (i.e. starts and ends in the home key), and we’ll talk about that next!
Ternary – ABA Ternary is made up of an A section ending in the tonic, a B section in the dominant or relative major, and then the A section again in the tonic. Remember the A section is often slightly varied the second time round, so don’t be put off by this when trying to identify the form.
Sonata (A1A2BA1A2) – Sonata is actually a complicated kind of rounded binary form! Like I said earlier, very few pieces fit the model perfectly, but there are three main sections: Exposition, Development, Recapitulation. It’s quite common for there to be both an introduction before the exposition proper and a coda (or closing frame) after the recapitulation at the end.
Exposition: Two subjects (or themes), one in the tonic, and one usually in the dominant (in the Romantic period it’s quite often a different key). Like simple and rounded binary forms, if it’s a minor key, it’ll probably be either the dominant (V) or the relative major (III) for the second subject/theme. The themes are usually contrasting, and especially in the Classical period, the second subject tends to be more lyrical than the first, which is often more strident.
Development: Does more or less what it says – the development usually transforms, or takes as a starting point both of the exposition themes and develops them. Another key feature (geddit?) of the development is that it modulates a lot. It’s also where you’ll find a lot of the more dramatic moments of the piece.
Recapitulation: Having undergone the development, the recapitulation, (or recap for short) usually (but not always) brings us what’s called the double return, that is we come back to the first subject in the home key. We then get the second subject, but this time also in the tonic. The recap is almost always great moment of arrival in the sonata after all the drama and modulation of the development. The recap is also quite often followed by a coda frame of some sort, whose job is to help give the piece closure.
Rondo – ABACAD…A – Rondo form alternates an A section, the ‘rondo’ which is in the tonic key, with new material, so the A section is a bit like a refrain. Sonata-rondo also looks a bit like this: ABACAB, where A and B are the first and second subjects, or themes.