Something happens in the third semester of music theory. Sometimes it’s early in the semester, sometimes it’s later in the semester, depending on which book (if any) you’re using and how the class is overall. Currently, it’s happening right now for my Theory III kids this year.
It’s the point where we’ve cleared most of the *conceptual* framework and can start getting more into analyses and study of entire pieces. There are still concepts to cover (extended chords, other modulatory techniques, etc.), but most of the puzzle pieces are in place. We can move into analysis that transcends the descriptive and start thinking in terms of prescriptive; that is to say, we can start applying what the analysis yields to actual performance.
For various reasons, most books/sequences use German Lieder as the springboard into this new analytical world. There are piano pieces, chamber works, and symphonic works that could show these concepts equally well, but it is almost always Lieder that we use. Part of this is because there are so many songs out there; you could get the entire third semester and a good hunk of the fourth just with four composers (Schubert, Schumann, Wolf, Schoenberg for the fourth). Part of it is because you can bring in text for discussion, taking things out of the realm of absolute music and into something more programmatic. And part of it is simply because it’s just so wonderful. Schumann’s cycles especially do such a great job at showing the totality of human emotion.
I leave you with a video. This is tenor Ian Bostridge and pianist Graham Johnson with what is my favorite Lied – Robert Schumann, Myrthen, Op. 25, no. 1, “Widmung.” Du meine Seele, du mein Herz…