Poetry by post seals friendship, explores nature of verse; Segues - A Correspondence in Poetry, by William Stafford and Marvin Bell. Boston: David R. Godine. 52 pp. $12.95 hardback. $8.95 paperback.

''Segues'' is written by William Stafford and Marvin Bell, two of America's most respected poets. Theirs is an ambitious experiment, and while the quality of the poetry is uneven, their book is a refreshingly novel event on the literary scene.

The poetry of ''Segues'' began in 1979, when the two poets taught together at a writers conference in Alaska. Despite the distance between their homes (Stafford lives in the state of Washington and Bell in Iowa), they decided to collaborate on a writing project to strengthen their friendship and explore the ways a poem comes into being. Stafford began the chain of ''verse letters,'' and each successive poem grew out of the subject, tone, or language of the previous one. A long-distance version of the ancient Japanese renga (linked verse), their correspondence was a form of mutual inspiration, making a poetic duet from what is usually a private solo performance. The title ''Segues'' is a term for the transitions inside a piece of music that allow one theme to grow into another.

Certainly more is required of a book than the novelty of the poems' call-and-answer progression to make any lasting impression on the reader. Of the 44 poems in the collection, only a handful are strong enough to stand on their own merits. Stylistically, this work is a marked contrast to many of the popular trends in poetry; they have none of the oblique, hard-edged, lines, vaguely surreal visions, or self-conscious absorption in the surface qualities of verse. The writing possesses some of the intimacy of letters and the give-and-take of good conversation - two practices nearly extinct in American social life. Stafford and Bell are conversational, even voluble, in their writing, paying most attention to the subject at hand and the emotional tone. At times the relaxed atmosphere of this correspondence brings out their worst tendencies, allowing slack, unfocused lines or thin philosophical pronouncements that dilute the poem's effect.

Stafford is a deceptively simple poet whose best work coaxes a sense of mystery and emotional eruption from the most familiar experiences. Much of his writing is word-generated, seeming to uncover its subject and mood as the poem progresses. In ''Segues,'' he has not only his own language as a starting point, but the memories and sidewise observations of his partner. Judging by the enthusiasm of the poems, he enjoyed the challenge. Bell's poetry usually relies on sudden twists of diction and focus. He adopts some of Stafford's simplicity here in autobiographical pieces about his family and childhood. By and large, these poems aren't as well crafted as his previous work, displaying less control but providing more emotional involvement.

But in a curious way, ''Segues'' is a better book than the sum of its table of contents. Some of the most interesting moments seem to occur behind and between the actual poems. There is the feeling of true correspondence in these poems - not just the ''letter'' variety but also the sympathetic reverberation between two visions. In the first poems, the triggering elements are concrete: Stafford begins with the word ''bell,'' which implies his correspondent's surname; Bell adds to this ''Belle,'' his mother's name. Then ''mothers'' and ''names'' become subjects for the next writings, and the path leads on. As they proceed, the poets find so much common ground, that a question or nuance from one seems to release a gust of energy from the other.

''Part of the time I want to tell you something so clearly you get there before I finish. I want you to know and be sure of more than I'm saying. . . .''

At the conclusion, the sense of culmination is more involved with the poets' bond than the poems' subject matter. But the reader is left with several strong impressions: first, that the book describes a greater human landscape than was initially apparent, implied more than defined by the individual poems in the way flares in a night sky provide glimpses of the broad countryside; second, a reaffir-mation that the greatest strength of modern poems lies not in the surface dazzle and linguistic effects, but in the way a work impels a reader's imagination to be involved with the creation of its subject; and third, as these writers discovered, that there is more common emotional ground in the literary arts than the caustic visions that are becoming the sanctioning elements of much contemporary writing.

Had the voices and outlook of Stafford and Bell not been so accommodating, there might have been a more dialectical tension to enliven this poetry dialogue. ''Maria Nephele,'' the book by Nobel Prize-winning poet Odysseus Elytis, where a young radical woman and an older man of letters duel in corresponding monologues, comes to mind. Of course, Elytis played both instruments in his duet and controlled the ultimate design of their pairing. The ''Segues'' poets, like improvising jazz musicians, had to listen and anticipate the direction of their composition. For the reader, the reward is an intimate glimpse into the work of two accomplished writers and the sense of participation in the unfolding of their poetry.