Recall that a sestina is a 39 line poem of six 6-line stanzas followed by a 3-line stanza. The 6-line stanzas have lines that end in the same six words, following this permutation pattern:
123456 615243 364125
532614 451362 246531
The final stanza uses two of the six end-words in each of its three lines. An original pattern for these was 2-5, 4-3, 6-1 but this is no longer strictly followed.
Can sestina-like patterns be extended to other numbers? Poet and mathematician Jacques Roubaud of the OULIPO investigated this question and he considered, in particular, the problem of how to deal with the number 7 of end-words -- for 7 does not lead to a sestina-like permutation. Rombaud circumvented the difficulty (see Oulipo Compendium -- Atlas Press, 2005) by using seven 6-line stanzas, with end-words following these arrangements: