2-line stanzas. A 2-line stanza is called a couplet. aa is a rhymed couplet; xx an unrhymed couplet. The French call rhymed couplets rimes plates or rimes suivies. 3-line stanzas. A 3-line stanza of any kind is called a tercet. aaa is a triplet. axa is pretty common, occurring in forms such as the villanelle and terza rima (where in both cases there are also rhymes between stanzas). The haiku is a species of tercet, and so are some varieties of englyn (though not the one featured on this site).