Modulation means a change of key.
Modulation has a reputation as a troublesome subject, difficult to master, and is invariably studied within the context of harmony. This is a pity, because the principles and techniques of modulation are not difficult to grasp, and apply equally to melody as well as to harmony. There are lots of options in melodic modulation too and, in a real sense, great freedom, because harmony does not need to act as a restraint.
Transposition is first on the menu. It is a very similar process to modulation that moves the pitch of a group of notes up or down. An accidental is a symbol in a score which modifies the pitch of a note and is used to signal modulation. Modulation type explains how to modulate and the underlying principles behind the different types of modulation. Parallel key modulation is a move to another key that shares the same tonic note as the current key. The circle of fifths is a versatile technique and is used as the basis for modulation to a relative key, close key or distant key.
This part of the guide begins the transition from melody to harmony. It mixes melody and harmony freely and shows how the two relate without going into the detail of how harmony works, which is covered in the next part of the guide.