1) The music theory (or the musicology) of classical music
This is the part of music theory that relates to music from the 17th to the beginning of the 20th century.
This "classical" music theory can be briefly summarized in the following seven points. (You can download a more detailed version of this list here as a PDF file - 86 KB.)
1. the intervals
2. the chords
3. the meter / the time signature
4. the rhythm
5. the scales / keys / tonality
The first five items mentioned form the basis of classical music theory. In addition, there are other areas that also belong to music theory, but which are so broadly diversified that they can also be regarded as independent areas: