O wild West Wind, thou breath of Autumn's being, / × × / × / × × / / Thou, from whose unseen presence the leaves dead × / × / / × × / × / (×) Are driven, like ghosts from an enchanter fleeing, As the examples show, iambic pentameter need not consist entirely of iambs, nor need it have ten syllables. Most poets who have a great facility for iambic pentameter frequently vary the rhythm of their poetry as Donne and Shakespeare do in the examples, both to create a more interesting overall rhythm and to highlight important thematic elements. In fact, the skilful variation of iambic pentameter, rather than the consistent use of it, may well be what distinguishes the rhythmic artistry of Donne, Shakespeare, Milton, and the 20th century sonneteer Edna St. Vincent Millay. Several scholars have argued that iambic pentameter has been so important in the history of English poetry by contrasting it with the one other important meter (tetrameter), variously called "four-beat," "strong-stress," "native meter," or "four-by-four meter." Four-beat, with four beats to a line, is the meter of nursery rhymes, children's jump-rope and counting-out rhymes, folk songs and ballads, marching cadence calls, and a good deal of art poetry. It has been described by Attridge as based on doubling: two beats to each half line, two half lines to a line, two pairs of lines to a stanza. The metrical stresses alternate between light and heavy. It is a heavily regular beat that produces something like a repeated tune in the performing voice, and is, indeed, close to song. Because of its odd number of metrical beats, iambic pentameter, as Attridge says, does not impose itself on the natural rhythm of spoken language. Thus iambic pentameter frees intonation from the repetitiveness of four-beat and allows instead the varied intonations of significant speech to be heard. Pace can be varied in iambic pentameter, as it cannot in four-beat, as Alexander Pope demonstrated in his "An Essay on Criticism": When Ajax strives some rock's vast weight to throw, The line, too, labours and the words move slow. Not so when swift Camilla scours the plain, Flies o'er th'unbending corn, and skims along the main. In the first couplet, in phrases like "Ajax strives", "rock's vast weight", "words move slow", the long vowels and accumulation of consonants make the syllables long and slow the reader