As we have seen, scansion is the act of mapping out stress patterns in order to ascertain the metre (rhythm). In the accentual-syllabic system, the dominant tradition in English, both accents (stresses) and syllables are measured and counted. In accentual metre, the stresses are counted and the syllables can vary. In syllabic metre, the syllables are counted, while the stresses can vary.
Here is pentameter, the line of five
That English poetry still keeps alive.
A poet may get a rhythmic ‘riff’ in his or her head, much the way a musician does. T.S. Eliot wrote that ‘a poem, or a passage of a poem, may tend to realize itself first as a particular rhythm before it reaches expression in words’ (Eliot, quoted in Harding, 1976, p. 87). Or the impetus for a poem may be the sound of a word. Both the language and the metre contribute to the overall mood of the poem: sad, ecstatic, sober, quirky, etc. If a poem with a regular metre varies at all, it still tends to be drawn back to its ‘home’ – the way a piece of music is usually drawn back to whatever key it's in. Until it does that, we feel uncomfortable, as if something is not quite right.