Common metre or Common measure, abbreviated C. M., is a poetic meter consisting of four lines which alternate between iambic tetrameter (four metrical feet per line, with each foot consisting of an unstressed syllable followed by a stressed syllable) and iambic trimeter (three metrical feet per line, with each foot consisting of an unstressed syllable followed by a stressed syllable), rhyming in the pattern a-b-a-b. It has historically been used for ballads such as Tam Lin, and hymns such as Amazing Grace and the Christmas carol O Little Town of Bethlehem.