https://poets.org/glossary/triolet
The requirements of this fixed form are straightforward: the first line is repeated in the fourth and seventh lines; the second line is repeated in the final line; and only the first two end-words are used to complete the tight rhyme scheme. Thus, the poet writes only five original lines, giving the triolet a deceptively simple appearance: ABaAabAB, where capital letters indicate repeated lines.
History of the Triolet Form
French in origin, and likely dating to the thirteenth century, the triolet is a close cousin of the rondeau, another French verse form emphasizing repetition and rhyme. The earliest triolets were devotionals written by Patrick Carey, a seventeenth-century Benedictine monk. Brit
How my heart sings 