Patrick Bahls and Richard Chess of the University of North Carolina at Ashville have organized a "Conference on Constrained Poetry" to be held on November 19-20 in celebration of the 50th Anniversary of OULIPO (short for French: OUvroir de LIttérature POtentielle), founded in 1960 by Raymond Queneau and François Le Lionnais. The group defines the term littérature potentielle as (rough translation): "the seeking of new structures and patterns that may be used by writers in any way they enjoy." Constraints are used to trigger new ideas and the Oulipo group is an ongoing source of novel techniques, often based on mathematical ideas -- such as counting letters and syllables, substitution algorithms, permutations, palindromes, and even chess problems.
The square stanzas in recent postings (November 14 and November 15) illustrate constraints that can shape poems -- but the square is, like the sonnet and sestina, an old poetic form that predates the Oulipo.
A popular Oulipian constraints is the snowball in which the number of letters (or syllables or words) changes by one as we move from line to line. (Snowballs also are featured in my posting for August 5.) Here is a "melting" snowball by Harry Mathews (a long-time member of Oulipo) from the Oulipo Compendium (Ed. Harry Mathews & Alastair Brotchie, Atlas Press, 1998).
In addition to blog postings mentioned above, these also have featured Oulipian ideas. August 23: "The Irrational Sonnet -- an Oulipian form" and March 25: "Queneau and the Oulipo."