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). 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".

Common metre is related to three other poetic forms: ballad metre, fourteeners, and common-metre double.

Like the stanzas of the common metre, each stanza of ballad metre has four iambic lines. Ballad metre is "less regular and more conversational" than common metre. In each stanza, ballad form needs to rhyme only the second and fourth lines, in the form A-B-C-B (where A and C need not rhyme), while common metre typically rhymes also the first and third lines, in the pattern A-B-A-B.

Another closely related form is the fourteener, consisting of iambic heptameter couplets: instead of alternating lines of iambic tetrameter and iambic trimeter, rhyming a-b-a-b or x-a-x-a, a fourteener joins the tetrameter and trimeter lines, converting four-line stanzas into couplets of seven iambic feet, rhyming a-a.

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