common metre

Common metre or common measure — abbreviated as C. M. or CM — is a  poetic metre 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. The metre is denoted by the syllable count of each line, i.e. 8.6.8.6, 86.86, or 86 86, depending on style, or by its shorthand abbreviation "CM". 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". The upshot of this commonality is that lyrics of one song can be sung to the tune of another; for example, "Advance Australia Fair", the national anthem of Australia, can be sung to the tune of "House of the Rising Sun".