Hylander, that is how I used to read iambic trimeter after you and mwh taught me how in the Ajax thread (for which I am very thankful!). However I read the introduction to Sidgwick's verse composition a few weeks ago, and found some advantages to his presentation.Iambic trimeters consist of three iambic metra, which (without permissible resolution of _ to υ υ) consist of x _ υ _. Iambic trimeters shouldn't be analyzed as iambic "feet" (υ _), but rather as metra. That's why they're called iambic trimeters and not iambic hexameters. Dactylic hexameters, however, can be analyzed in terms of dactylic "feet."
First, here are his rules, more or less:
1. Sidgwick explains iambic trimeters as a series of 6 iambic feet (u _) with substitution to spondees (_ _) possible in feet 1,3,5.
2. Substitution to tribrachs (u u u) in feet 1,2,3,4,5 are possible with the limitation that the tribrach can't be broken after the second syllable only, otherwise it would sound like a trochee (_ u) rather than a iambus.
3. Substitutions to dactyls (_ u u) and anapaests (u u _) are possible, with more limitations in tragedy than in comedy. Anapaests can only occur in the first foot in tragedy and are never broken. Menander, writing comedy, uses anapaests in feet 1-5 though, and frequently.
4. Dactyls, being a resolved spondee, are only possible in feet 1 and 3 in tragedy, but Menander uses them in foot 5. Obviously a dactyl can be broken after the first foot and the resolution still works. But not after the second, otherwise it's no longer a resolved spondee.
5. A caesura is necessary in foot 3 or 4.
6. A quasi-caesura is permissible (elision at the end of the 3rd foot).
7. Foot five cannot be broken (rule of the Final Cretic).
The anceps formula is visually simpler to process, and far less complicated to write down, of course, but I just can't hear it with my ear. The above I can hear, if that makes sense.