Scansion is the process of marking the stresses in a poem, and working out the metre from the distribution of stresses. The verb is to scan. 'Mark' can be taken to mean both 'notice' and 'annotate', the latter often done with a u for an unstressed syllable and a slash, /, for a stressed one.

Patricia Beer's poem 'The Conjuror' might be taken as an example. The first line has stresses falling thus: "aRRIving EARly AT the CEM e TERY", or u/u/u/u/u/, which sets up a clear pattern, | u/ | u/ | u/ | u/ | u/ |, an iambic pentameter. The next has clear stresses on "one", "clock", "looked" and "round", which is only four at first glance, but there is also a lighter stress on the "for" at the start of the line, particularly as the following "the" is less stressed. With x being used as a 'missing' syllable - like a rest in music - this line can be scanned as | x/ | u/ | u/ | u/ | u/ |, still maintaining the iambic pentameter. The third line, however, introduces a variation, holding back its first stress for an extra syllable - "at the last sparks", which can be scanned | uu | // |, after which the iambs pick up again until the end of the stanza.

What this process achieves is a diagrammatic representation of the metrical effects of a poem. To see Beer's first stanza displayed thus

| u/ | u/ | u/ | u/ | u/ |
| x/ | u/ | u/ | u/ | u/ |
| uu | // | u/ | u/ | u/ |
| u/ | u/ | u/ | u/ | u/ |

demonstrates its regularity and variations, and helps a reader or listener understand why those "last sparks" are so central to this stanza - the moment of irregularity within what is otherwise regular makes them stand out for the ear.

By contrast, scanning Alan Brownjohn's 'Incident on a Holiday' reveals that, although he largely eschews a regular foot, he does maintain a five-stress line in the first stanza, and in most of the poem, thus giving the poem something of the irregular rhythms of prose, while the accentual metre simultaneously keeps a form of regularity.

Some poems, such as D J Enright's 'Dreaming in the Shanghai Restaurant', avoid even accentual regularity. Note, though, that this poem about agreeable balance makes a kind of music out of the sentences, often balanced agreeably around a semi-colon, instead of the syllables that scansion measures.

Edwin Morgan's 'Song of the Loch Ness Monster' presents a great challenge to most attempts at scansion.