[Bill Carpenter]

Hi Perry,
All kinds of good responses here. I just wanted to put my two cents in in response to your comment that an IP line should have five evenly spaced stresses. That is only half true. You can think of the Platonic IP line as a sort of accompaniment that you play your instrument both with and against. Some poets hew very close to it and can achieve great charm that way, creating an atmosphere of steadiness and meditation -- at one with the invisible companion. (Sphereans will recognize the poet.) Since there are infinite degrees of stress and unstress (poets frequently use a 4-point scale instead of the binary on-off) and in syllable length, even verse close to the Platonic form can be rich and uniquely flavored if it avoids crude monotony. Some poets freely substitute. Well-recognized substitutions include initial trochees, initial headless iambs, initial or medial double iambs, medial trochees and spondees, and anapests. There are traditional rules regarding substitutions that you can read about. You generally do not want to lead the reader out of a double meter and into a triple meter by using too many triple meter substitutions, if you want to write recognizable IP. As the poets here have said, you can do what you like, but as Wallace Stevens said, It Should Give Pleasure, so the approach should be consistent for the poem as a whole.

Since you asked this question, you would surely enjoy Tim Steele's "All the Fun's in How You Say a Thing," a delightful treatise on meter in poetry. Bill

PS. I guess I should add that the accompanist is only there if you summon him/her with a sufficient invocation in the first couple of lines.
PPS. Following up on Aaron N.'s comment, we not only hear different kinds of equality but we often try to search it out or solicit it, compressing one line and stretching another to give lines approximate equality in length, isochrony.