Of all the new musical forms that developed in the second half of the eighteenth century, sonata form is the most important. In fact, it is arguably the most important musical development of the past thee hundred years or so.
Keep in mind: sonata form and a sonata are NOT the same thing.
As you recall from your study of the Baroque era, a sonata is a musical composition for unaccompanied solo instrument, or for one or two solo instruments with keyboard accompaniment. It’s usually written in three (or sometimes four) contrasting movements.
Sonata form, on the other hand, is a specific form in which the first movement of a multi-movement work (and sometimes other movements as well) is written. In the Classical period, sonata form was used for the first movements of sonatas, symphonies, concertos, and string quartets.
(Keep in mind, also, that the great composers of the Classical era who pioneered this form didn’t think of it as a specific technique with a name. In fact, it was dubbed sonata form by later musicologists who studied it: they looked at Classical-era sonatas and noticed that their first movements were structured according to this technique, and so, just to confuse you, they decided to call it sonata form.)
It is sometimes called sonata-allegro form, because the first movement in a multi-movement Classical work is usually marked allegro, i.e. to be played at a fast, lively tempo.
Think of sonata form as a narrative arc in a story, which we can express as a map:
In a work of literature like a story or play, the author introduces the principle characters in the first few scenes. Shortly thereafter, the main conflict is introduced. The principle characters struggle with and against the conflict through a series of scenes of rising action and drama. The narrative comes to a climax, and then resolves. The end.
In sonata form, the “characters” are melodies and keys. A map of sonata form looks like this:
The narrative arc of a story is structured as Beginning-Middle-End. In sonata form, those elements are called Exposition-Development-Recapitulation.
In the beginning is the exposition. Remember how the main melody in a fugue was called a subject? In sonata form, it is called a theme. The composer introduces this theme, which we will cleverly call Theme I. Theme I is in the key of the piece, i.e. the tonic. In other words, if the piece is called “Sonata in C Major,” Theme I is . . . in C major. We’ll call it Key I.
Then the composer introduced a second theme, which we will cleverly call Theme II.
Theme II, the second melody (or, in literary terms, the second principle character), is usually in the dominant — the fifth scale degree of the tonic. Therefore, if the piece is in C major, Theme II will be in G, the V or our I. (As you may recall, the vast majority of music in the Common Practice Period follows a similar harmonic progression (or, in literary terms, you can think of it as a similar narrative arc): I-V-I.) We’ll call it Key II.
If a piece is in a minor key, however, Theme II will typically (though not always) be in the relative major, i.e. the third degree of the scale.
We got there through modulation. To modulate, or change from one key to another, the composer usually relies on a harmony that is shared by the keys of both Theme I and Theme II. This modulation happens in a brief transition or bridge, where the melody of Theme I shifts into the new key and Theme II is introduced in that key.
So now, we are in Theme II, in the new key, Key II. Theme II introduces different melodic ideas into the piece; its feeling/content/atmosphere usually contrasts with those of Theme I. The composer will emphasize the new key area, riffing on the new melodic material, and tricking your ear into thinking that key II is home base. But it is not. Just like in baseball, or in the Wizard of Oz, we eventually have get to home. And home is actually key I.
How do we get there? We need to traverse the area of greatest conflict and drama, the development section. In a story arc, this section of sonata form is the equivalent of the rising action leading to the climax. In the development section of sonata form, we begin to hear fragments of both Theme I and Theme II, played with, toyed with, messed around with, cycling through many keys. This is the point in the movement of the greatest tonal instability. It creates dramatic tension in the music, leaving the listener unsettled, and possibly asking him- or herself one or more of the following questions:
In order to give the listener a longed-for sense of repose and relief, the composer then kindly consents to bring the piece back into the home key. This is done by a reintroduction of Theme I in Key I, called the recapitulation. And just for good measure, the composer will trot out Theme II again, but this time played in Key I — just so you, the listener, know you’ve finally made it home. The dramatic tension has been resolved. There follows a brief section called the coda (meaning “tail” in Italian), which is a sort of summing up of what’s gone before, allowing the listener to think back on what has transpired musically, from the comfort of the home key.
This video of Mozart’s Piano Sonata no. 16 in C Major shows what happens in sonata form.
And this is a wonderful little song, which explains it all to you.