Canonic imitation is important for the new organist to understand.
A 2-part canon is a musical passage with a follower and a leader; in such a passage, thematic material follows itself note for note in another voice at a certain short interval of time, most often at the interval of a 4th, 5th, or octave, but can appear at any interval.
A drone (held bass note), or perhaps one or two more free voices (using "sweet" intervals like 3rds and 6ths which don't interfere with the ear hearing the follower and leader), could be added to this 2-part canon to create a chordal harmony.
For a number of examples incorporating this type of writing, the reader may consult this author's scores and their suggestions for performance found on this web site [See menu bar, Free Stuff].
Here's an analogy: it's like watching a jet flying along with another one trailing very close behind, without having a collision.
It's an even trickier business when one or the other is flying upside-down, as in the 10th variation from this author's Op. 4 Variations on a cantus firmus in C major [See menu bar, Free Stuff].
(con't in Part II)