The definition is given in Chapter IX (on page 84).
A fair illustration of the utility of the term "Group of Parts" is seen in Schumann, op.

68, No.18.

Others will be cited in the following Lesson.
LESSON 11 .-- Analyze the following examples of the enlarged Three-Part Song-form.

As before, the form of each Part should be defined, and introductions and codas (if present) properly marked.

All of the given examples belong to this chapter, but are not classified; it is purposely left to the student to determine where repetitions occur, and whether they are exact, or variated,--in a word, to decide which of the above diagrams the composition represents.
Mendelssohn, Songs Without Words, No.