The fugues in this book are, for the most part, quite different from each other. However, all pieces that are called fugues (or fugatos or fughettas) have one quality in common, imitation. A fugue almost always starts with a melody (referred to as the subject) in one voice, that is imitated by the other voices as they enter. The whole piece might consist of one imitation after another, or there may be alternating sections of imitative and non-imitative music.