There are several variables in the field of Canons, in both the Worlds of TIME and of PITCH, which will affect the final result in its Rhythm, Its Harmony, and of course, in its Melody.

Types of canons

The specifications of "Forward", "Similar", "Same size", and "Same pitch",
               being more fundamental and evident,
          are usually understood and not mentioned,
     and only those of "Retrograde", "Inverse", "Augmentation", and "Interval distance"
          are specified.

In Time

Forward or Retrograde
Almost all canons answer "forward" because TIME is a uni-directional World,
     and Retrograde Canons, which answer backward, are not really perceptible,
               especially if they are long,
          because the ear cannot easily perceive what is played backward,
               especially at a distance (in time).

Same size or Augmentation
Though most canons answer with the "same size" note-values,
     an answer in longer note-values (by Augmentation)is easily perceptible
          because the melody remains substantially the same,
               merely played slower (usually twice as slowly).
     Dependng on who starts, the answer might be by diminution rather than by augmentation.

In Pitch

Similar or Inverse
Traditionally, as we well know, most canons answer in similar motion
          because the melody is far more easily recognized in this form.
     However,Natural Swing Canonsare, by definition, in inverse motion,
          and are far more easily recognized than Retrograde Canons,
               because the answer is always close (in time) to the original statement.
      Face to Face Canonsare both Inverse and Retrograde.

Same pitch or Interval distance
Most simple canons, likeRoundsorEvolving Canons, answer (enter) at the same pitch,
     but,Natural Circle Canonsanswer
          at the fifth (or fourth), if the answer is on the next chord, or
          at the second (or seventh) if the answer is 2 chords away.
     In learned, classical canons, writing at theInterval, especially over aCantus Firmus,
          was once very popular, and most successful in the works of J. S. Bach.