Every piece of music has an overall plan or structure, the “big picture”, so to speak. This is called the form of the music. Form is found all around us and provides our world with a sense of structure or organization. Form becomes very important with large pieces of music. Below are descriptions of some common musical forms:
AB
AB (or Binary) form consists of two parts, usually designated A and B. Often these sections repeat.
ABA
ABA (or Ternary) form is a three-part form. The B section is different from the A section. The B section may contain new melodies, different instrumentation or may be in a different key. A sections are usually exactly or nearly the same as each other.
Rondo
Rondo form follows an A-B-A-C-A sequence. There are sometimes variations on the A-B-A-C-A plan. Sometimes it will appear A-B-C-B-A or in another sequence.
Theme and Variation
Theme and variation is one of the simpler forms. Music in this form may say “Theme” at the head of the A section and then list a variation number over each of the other sections. The theme is usually a self-contained section that ends with a double bar line. Often, the theme is in binary form. Each of the following variations restates the theme in a new light. Usually, if the theme is in a major key, there will be a variation in a minor key. There may also be variations in rhythm, instrumentation, or tempo.
Fugue
A Fugue is a form that can be compared to a round, but is much more intricate. It is a composition in which the basic principle is imitative counterpoint of several voices. That means that a theme, also called a subject, starts in one instrument then is stated one by one in each remaining instrument. The lines of music are constantly in motion, weaving in and out of each other, creating what is known as counterpoint. Each line of music creates harmonies with the other, yet keeps its independence.
Sonata
Another type of ABA form, Sonata form, is made up of three basic segments: exposition, development, and recapitulation. Sonata form, however, also may include an introduction. The introduction is optional, and often is very short. If a composer chooses to extend the introduction, it is often slower than the exposition. It may or may not contain material which is used later.
Sonata Details
The primary thematic material is found in the exposition. This section can be divided further. A first subject group, which may be one or more themes, will be introduced. These themes are all in the home key of the piece. For example, if the piece is in C Major, all of the music in this group will be in C Major.
There will then likely be a transition in which the composer modulates to a new key. In the second subject group, one or more themes in a key different from the first subject group will be introduced. The material of the second subject group is often different in rhythm or mood from that of the first group. Frequently, the material is more lyrical.
Following the second subject group there may be a codetta, which brings the exposition to a close.
The development section follows the exposition. It often starts in the same key in which the exposition ended. Development sections often move through many difference keys. It will usually consist of one or more themes from the exposition, but now altered. Alterations include taking material through distant keys, breaking down themes into their smaller motifs. Sometimes new thematic material may be introduced. Development sections vary in length from piece to piece. Regardless of its length, it almost always shows more harmonic and rhythmic instability than the other sections.
The final section in Sonata form is the recapitulation. The recapitulation is an altered repeat of the exposition. It contains the same first subject group, and in the same form as it was in the exposition. The transition material does not usually change key. The second subject group is also stated again, but may be in a different key than it was in the exposition.
The piece may continue with a coda, which will contain material from other portions of the music. Like the introduction, a coda may vary in length.