The lengthening of a phrase, whether internally or externally, beyond its expected duration. "Expected duration" is defined contextually, and it may rely on such factors as: era, genre, and/or pre-established models. The expansion may occur either within a single group (stretching) or by stringing together (or appending) additional groups (adding). Stretching and Adding can also occur at the same time.
The most common way of identifying expansion is to compare it to a model from the same piece. For example, in a parallel period, if the first phrase is 4 measures but the second is 7 measures, there is likely to have been a phrase expansion because the first phrase established a model of length and the second phrase defied that expectation. This can happen when a phrase comes back later in a piece too, like a chorus that was 8 measures the first time but is now 10. The model for expectation can also come from the genre or style of music. If, for example, every verse you have experience with in bluegrass music was 8 measures, but a new piece in that genre has a 9-measure verse, it will likely feel like an expansion and then you can dive deeper in order to identify where an expansion might have occurred. Keep in mind that the repetition of an entire phrase is not typically considered an expansion technique. For example, if the first verse was 8 measures and the second verse was 16, if the 16-measure one was just the original 8 measures and its repetition, there is no expansion.
Also keep in mind that expansions are typically something you aurally experience first. Use the examples below to start developing a sense of what expansion sounds like.
Phrase expansion can happen within the bounds of a phrase or it can happen before the phrase starts and/or after the phrase ends. When it occurs outside of the phrase it's called an external expansion and it's the same thing as an external auxiliary section (prefix & suffix). This page only focuses on internal expansions.
Internal expansions come in many forms but they can generally be broken down into two major categories: (1) Adding and (2) Stretching.
Genre: Most dance music that I'm familiar with does not have internal expansion. It happens occasionally, but it is not common. European classical music employs it fairly regularly, but again, it is not common in dance music even in European classical styles. Many pieces simply don't have expansion.
An expansion created by the combination of repeated, additional, or unexpected groupings. There are three sub-categories of additive expansions:
Stretching occurs when a single grouping unit has been lengthened beyond its expected duration by increasing the length of one or more of its harmonies. This can happen in the following ways: