sonata-allegro form, though, is much more elaborate, and we hear differentiation of formal functions within subsections.Within the Exposition we can identify four subsections: Principal Tonal Area (P): This is the music we hear in the tonic key, which establishes the tonic key.Transition (T): This is the music that moves us away from the tonic key and toward the secondary key (normally, Dominant or Mediant).Secondary Tonal Area (S): This is music in the secondary tonal area: normally, the key of the Dominant (V), in a major key; the keyof the Mediant (III), in a minor key.Closing section (CL): This is music that brings the Exposition to a close in the secondary key, and the music itself normally signals to us that we’re reaching the end of the Exposition.Btw, You will not be expected to identify these subsections on your own. (But wouldn’t it be great to discover that you can hear them!) In this piece, which is in G major, the Principal Tonal Area (mm. 1-18) is characterized by a series of brief themes—phrases, if you will—each of which is clearly affirming the tonic key.Let’s listen with the score to mm. 1-18.1The Transition (mm. 18-27) is signaled by changes in dynamics and texture on the downbeat of m. 18. We also have a phrase elision on the downbeat of m. 18. The last phrase of the Principal Tonal Area is concluding with an Authentic Cadence on the downbeat of m. 18, even as the Transition begins at that same moment.