All Alexander Pope's significant poetry is written in what is usually known as heroic couplets: five foot lines rhyming in pairs and with each foot containing the iambic structure of an unstressed syllable followed by a stressed syllable. There are occasional variations on this pattern, but they are rare and, when they do occur, Pope clearly expects us to measure them against this norm. The immense variety of movement and tone that Pope achieves in his poetry is nearly always a matter of working within the bounds that the couplet imposes, rather than a matter of breaking away from them. His poetry illustrates the traditional paradox that one moves more easily for being in harness, or, to borrow the lines that Pope himself wrote in An Essay on Criticism when describing the strife between wit and judgment. The heroic couplet had been used in English poetry long before Pope perfected it.