"Helen" is written in free verse. It has no regular rhyme scheme or meter. That doesn't mean that it's a crazy mess, though. Few good poems are really crazy messes, even if they are in free verse. And H.D. was never one to play it fast and loose with a line. The poem is actually held together by the repetition of sounds (check out the "Sound Check" section) and more than a hint of the centuries-old blazon form.
What's a blazon? We're so glad you asked. A blazon is basically a list of a lovely lady's body parts in a poem. (Poets like Spenser dug blazons; Shakespeare liked to make fun of them.) A blazon will catalog and praise the parts of a woman's body—her shining eyes, her rosy cheeks, her voluptuous curves.
Like Shakespeare before her, H.D. plays with the traditional blazon form, and turns it inside out. Instead of enumerating and praising Helen's body parts, the speaker of "Helen" enumerates them and disses them (on behalf of "all Greece"). By the end of the poem, Greece sees Helen as a white, cold, still, deathly statue. Yeah, she's not exactly a bombshell anymore. And the result of this messed-up blazon is that something beautiful—Helen—turns into something ugly—death. Greece contorts the traditional blazon form, just as it contorts Helen's beauty, which means that H.D. is oh-so-cleverly weaving together her form and content in "Helen."