in a poem is "The artistic arrangement and use of the material (aural and visual) aspects of words into particular repetitive and/or serial forms as a means to structure a poem," then variation
, pattern's partner in crime, is
The artistic breaking of a pattern within a poem to create degrees of emphasis.
Once a pattern has been established, it may be varied. The effect is always the same: it produces emphasis. The degree of emphasis is directly related to the degree of variation. An extreme variation from the pattern will produce extreme emphasis, minor variation will produce minor emphasis, and any degree within the extremes is there to be played with. If a given word or phrase is of import in the poem, Then variation can be used to set it apart. Whether a phrase should get such emphasis is an important poetic question, and one that we should never tire of asking.
So pattern and variation are two primary tools in Apollo's toolbox. In a poem the material aspects of the words(both aural and visual) are being controlled. Taking a largely chaotic, Dionysian line or sentence, we begin to give it shape, and that shape is largely defined by the patterns and variations that we set into those words.