The triad is simply a poem consisting of three tercets, each
describing a related object or theme. A tercet is a stanza
consisting of three lines. Historically, the tercetís first and
third lines rhyme, leaving the middle line unrhymed. However, in
modern times the rhyme scheme has been abandoned (as have so
many other traditions), and the lines do not have to rhyme. The
traditional form is now called the envelope tercet, as the rhyme
scheme envelopes the middle line.