Ternary is a musical form consisting of three distinct sections. As with most musical forms, ternary involves large-scale repetition and so ternary form is typically defined as ABA because the main section (A) returns after the contrasting section in the middle (B). Often each section is repeated as indicated in the diagram below. Though it might seem logical, ternary form is not typically defined as being ABC. ABC is more often considered as part of the through-composed category because each section contains different music.
While the contents of each section can vary greatly concerning phrase and form, each section is commonly comprised of multiple phrases and very often those phrases combine together into a named form (very often a binary form). A ternary form is considered to be compound (a.k.a. composite) if one or more of its sections is comprised of a complete musical form. If a section does not contain a complete form, it can be called simple. In many compound ternary forms (minuet & trio and scherzo & trio in particular), all sections contain complete forms (often rounded binary form). In compound ternary forms of the 19th century, however, the last A section is often shortened and is simple, not compound.
The second section of a ternary form (contrasting section), usually referred to as the B section, is expected to provide contrast with the A section that preceded it. This contrast may come from a variety of musical domains including key, mode, texture, time signature, rhythmic ideas, melodic ideas, range, instrumentation, register, and so on. The length of B, however, is expected to be generally proportional to that of A.
In some genres (dances like minuet & trio for example), the A and B sections contain a relatively similar level of stability because they start and end in the same key and contain phrases that are usually tight-knit. In other genres (arias in particular), the B section is often less stable than A. B’s instability is largely due to starting and ending in different keys and a generally looser phrase-structural organization than A.
As with other forms, each section can be described in terms of being harmonically open or closed. However, it is most common for the A section to be harmonically closed and the B section might be either open or closed.
Modulation is possible within each section but is very rare in the A section and should be considered atypical when found there. If the ternary form contains a modulation within one of the main sections (A or B), it much more likely to occur in the B section of ternary forms found in arias but is not associated with dance forms like the minuet and trio.
Like other forms, ternary can contain auxiliary sections. Small transitions, small retransitions, small prefixes, and small & large suffixes are common. See the chapter on Auxiliary Sections for more information.
Chopin's polonaise in A major (Op. 40, No. 1) is a good example of a compound ternary form. Note the information on the score like the key-signature change in m. 25 and m. 81 and the use of internal repeat signs within the A and B sections.
The A section contains a complete rounded binary form. Compare measure 1 with measure 17 to see when the opening material returns in the middle of the A section's second reprise. The B section also contains a rounded binary form. Compare m. 25 with m. 65 to see when the opening material returns in the middle of the B section's second reprise. Notice that the first reprise doesn't have repeat signs and instead its repeat has been written out (compare mmm. 25-40 with 41-56). When the A section is repeated in mm. 81-104, the repeats that were in the original statement are now removed but otherwise the music remains the same. Many times, composers will simply write "da capo al fine" or "D.C. al fine" at the very end of the B section and not rewrite the A section. Instead, the instruction "da capo al fine" simply means to turn back to the "head" or beginning of the piece and play until the word "fine" appears, which you can expect will be found at the very end of the A section.
Take note of the stability of both A and B sections. As is common of classical dances that are in compound ternary form, both the A and B sections are relatively stable and self contained. If you didn't know the piece and someone played only the B section, you would like feel that it was a complete piece by itself.
"Waft Her Angels" from Handel's oratorio Jeptha is a simple ternary form and differs from the Chopin example in two notable ways. The first is the consistent presence of prefixes and suffixes. In Baroque arias like this one, this type of recurring material is known as a ritornello and it is a common framing device used either before, after, or before and after the singer's phrases. The second is the presence of a small retransition between mm. 29 and 30 that reintroduces the key of G major, the work's home key. Like the Chopin, this aria also has a stable B section, though B sections in arias are often untable instead.