Sonnet
A fixed-form fourteen-line poem — closed by a final turn — in Petrarchan or Shakespearean arrangement.
The sonnet is Western poetry's most durable fixed form: fourteen lines, arranged in the Petrarchan tradition as octave plus sestet, and in the Shakespearean as three quatrains plus a couplet. Rhyme schemes vary by tradition; what doesn't is the turn — the volta — that pivots the poem, and the pointed ending that crowns it.
Its structure is an argumentative mold as much as a metrical one: the opening sets (a scene, a case), the close disposes (reversal, widening, resolution). From Petrarch to Shakespeare's couplets to Rimbaud's "Le Dormeur du val," the constraint has never stopped proving fertile: fourteen lines, and everything must fit.
Example
"Shall I compare thee to a summer's day?" — three quatrains, then the sealing couplet.