Canon is a technique of the classical music and is a melody that is repeated exactly by a different part, entering a short interval after the original part. If the melody of the follower is the same as the original melody, it will be unison canon. If the top and the bottom of the interval is reversed, it will be inverted canon. And if the melody is played backwards, it will be retrograde canon.

The follower plays exactly the same melody as the leader. The canon of famous Pachelbel is included in this category. If the end of the canon is connected smoothly to the beginning, it will be called circular canon.

The follower begins on a pitch other than the starting pitch of the leader. For example, the pitch of the follower may be widening out. The follower may be adjusted to stay in the same key as the leader or it may be performed in a total different key.

This is the playing of a melody forward and backward at the same time. The follower plays the melody of the leader backwards. (So, you may be confused which is the leader and which is the follower.)

The follower plays the melody in the same pitch as the leader, but it plays the melody backwards. (The pitch will be adjusted to stay in the key.) Because of that, the follower begins with fifth or fourth pitch of the leader. It is said that many of Bach's canons are categorized in this field.

The follower plays the melody of the leader backwards. But the pitch will not be adjusted to stay in the key.

The follower plays the melody of the leader with enlarging or shortening the sound. For example, the follower might play the melody of the leader at half, twice, or three times, the speed of the leader. The follower starts at the same time or at different times.

It repeats the melody of the canon with changing the pitch.

This is a canon that has 2 leaders and 2 followers.

Morzart Canon	You can download MIDI files from here.
Pachelbel Street	This site has information about canon.