What Bob calls the ionic and Tim Steele calls the double iamb, and I call the pyrrhic/spondee, is a metrical flourish I just adore. I do it well: "We grieve for the twelve trees we lost last night," is the first line of "Track of the Storm," which Sam Gwynn used in the Penguin Pocket Anthology of English Poetry. Employing such a variation in line one is a way of saying I know what I'm doing, which Frost does all the time. Yeats uses it to spectacular effect. Among contemporary metrists Wilbur is an absolute master of the ionic. Let me add that Mezey is no slouch either. From "Teadance at the Nautilus Hotel": "In the curled bluish haze of panatellas," and "Of the old-fangled dance tunes."

Why do I love the ionic? Because IT ROCKS!, imparting a forceful swing to the verse when it is employed expressively and effortlessly.

Just popped in for a minute to pass along a useful tip. I've been rereading John Thompson's book, THE FOUNDING OF ENGLISH
METRE (which must be one of the best doctoral dissertations ever written). It was lavishly praised bu Ransom and Lowell, among others. You probably couldn't find anything much more illuminating on the subject of this thread. It is, as John
Hollander says in his introduction, "a pioneering study...of
the confluence of the phonology of English speech and the paradigms of of written verse structure." Another passage from Hollander's intro, which sums up neatly the whole matter:

The long life of English iambic verse from Surrey through Wallace Stevens and after has continued to be a full one, rich in ever-renewed rhythmic incident. As Thompson suggests, this is occasioned by the complex relation between metre--con- sidered as an accentual-syllabic paradigm--and the actual phonological rhythm of any utterance framed in such a paradigm... This book characterizes that relation as one of tension.

Ciao