In addition to the core sections of a work (i.e., sections called A, B, primary theme, secondary theme, refrain, episode, and contrasting middle/development/digression), other sections may introduce, follow, or come between these core sections. These sections are called auxiliary sections and there are two general categories
External Auxiliary Sections either introduce a piece/section (prefix) or follow the piece's/section's generic conclusion (suffix). Prefixes and suffixes come in small and large varieties.
A prefix (Rothstein, 1989) refers to music that comes before the generic start of a phrase or piece and tends to express a formal sense of "before the beginning." A prefix can be described as either small or large depending on whether or not it contains a complete phrase. Large prefixes contain at least one phrase and small prefixes don't have complete phrases and are typically far less noticeable. Small prefixes are often nothing more than the upcoming section's accompaniment which precedes the actual start of the phrase's melody and they may precede any phrase in a work. The most common type of large prefix is called an introduction. Introductions are often in a slower tempo than the rest of the work and often contain their own thematic material. Small prefixes can be found in works of most genres and eras, but large prefixes are less ubiquitous and tend show up more often in particular genres, like the opening of a symphony. Though, other genres like the piano sonata (Beethoven's "Das Lebewohl" op. 81a), string quartet (Mozart's "Dissonance" quartet), and dance forms like ragtime (Joplin, "The Entertainer") can contain them as well.
A suffix (Rothstein, 1989) refers to music that comes after the close of a phrase or piece and tends to express a formal sense of "after the end." The distinction between large and small again concerns whether or not it contains a complete phrase. Small suffixes can be found after the close of any phrase but the affect is quite different depending on the type of cadence they follow. After an authentic cadence they typically project a sense of stability and closure but after half cadences they tend to prepare for the entrance of the upcoming section and therefore project a sense of instability. Though possible after any phrase, large suffixes typically appear in two specific locations: (1) at the very end of a piece in the form of a coda, and (2) as the closing section of a sonata form's exposition and recapitulation. In most cases, suffixes contain musical material that is different than the phrase it follows though that material may be derived from an earlier phrase.
Generally, a section of music that functions to connect two core sections. In particular, a transition comes between two sections where the upcoming section is not the initiation of a large-scale return (i.e., between A and B, not between B and A). Transitions usually help to lead away from the piece's main section toward a contrasting section. Often a modulation is introduced to help prepare a section in a new key, though a modulation is not required. A transition also plays a role in the balance of stability and instability in a work. Core sections of a work are very often stable thematic statements (relatively), but transitions typically introduce instability (and a gain in energy) which will likely be countered by the stability of the section that follows.
Like suffixes and prefixes, transitions and retransitions come in "large" and "small" varieties. A transition can be described as either small or large depending on whether or not it contains a complete phrase. Large transitions contain at least one phrase and small transitions don't have complete phrases and are typically far less noticeable.
Near their end, transitions (and retransitions) often drive toward attaining the dominant chord of the upcoming key. Often, a suffix will begin once the dominant has been attained in a situation sometimes called standing on the dominant (William Caplin) and dominant lock (James Hepokoski and Warren Darcy).
A transition may have a clear stopping point before the next section starts, or there may be a single melodic line that fills the space between it and the upcoming section, or the transition my end at the onset of the new section with an elision. In more vague cases—the end of the transition and the start of the new section may be hard to pinpoint but still clear that it must have happened during a particular span of time.
Transitions are commonly found in sonata forms between the primary and secondary themes and in rondo forms when the A section (a.k.a. refrain) is being departed from to prepare for contrasting sections (a.k.a. episodes). Small transitions are often found in ternary forms to connect the A and B sections.
A retransition is very similar to a transition but its location and function are different. Retransitions come between two sections where the upcoming section is the initiation of a large-scale return (i.e., between B and A, not between A and B). In most cases, retransitions help prepare the return of the piece's main section. In a ternary form this would be the A section, in a sonata form this would be the restatement of the primary theme at the onset of the recapitulation, and in a rondo this would be the return of A section (a.k.a. the refrain). A retransition often drives toward attaining the dominant chord of the home key and will often prolong the dominant once attained, usually in the form of a suffix. Retransitions may have a clear half-cadential ending (possibly followed by a suffix), or they may have an elided ending that coincides with the initiation of the following section.