“Ode on a Grecian Urn” consists of five ten-line stanzas, each of which is written in iambic pentameter. (Recall than this metrical form features five iambs per line, where an iamb consists of one unstressed and one stressed syllable, as in the word “to-day.”) Iambic pentameter was arguably the most common meter of Keats’s day. It was popular among poets because it could approximate the cadences of natural speech without lapsing too much into the lilting rhythms common in songs. The five-beat line had an intrinsically noble sound that served particularly well in poems with serious subject matter. Hence, it was the preferred verse for epic poems like John Milton’s Paradise Lost, which Keats himself admired tremendously. Yet despite his relatively regular use of iambic pentameter in this poem, Keats also introduces a great deal of metrical variability within each stanza. The primary form of metrical variability comes from the substitution of individual iambs for other types of feet with alternative rhythmic patterns. This strategy of substitution introduces unexpected cadences, which prevents the speaker’s address from becoming too predictable and hence contributes to the overall richness of the poem’s language.

To get a sense for the effect of this metrical variability, let’s analyze the poem’s opening stanza (lines 1–10):

Thou still / un-rav- / ish’d bride / of qui- / et-ness,
       Thou fos- / ter-child / of sil- / ence and / slow time,
Syl-van / his-tor- / i-an, / who canst thus / ex-press
       A flow- / er-y tale / more sweet- / ly than / our rhyme:
What leaf- / fring’d leg- / end haunts / a-bout / thy shape
       Of de- / i-ties / or mort- / als, or / of both,
               In Tem- / pe or / the dales / of Arc- / a-dy?
       What men / or gods / are these? / What maid- / ens loth?
What mad / pur-suit? / What strug- / gle to / es-cape?
               What pipes / and tim- / brels? What / wild ec- / stas-y?

Out of these ten lines, six feature perfectly regular iambic pentameter: lines 1 and 5–9. Each of the four remaining lines has at least one metrical substitution or other rhythmic oddity. In some cases, the variation is quite subtle, and we might not even notice it if we’re attempting to make the poem conform to iambic pentameter. Line 4, for instance, only conforms to regular iambic rhythm if we elide the final two syllables of “flowery” into a single syllable: “A flow- / ery tale.” A more significant variation appears in line 2. This line contains substitutions in the final two feet, where a foot with two unstressed syllables (known as a pyrrhic) is followed by a foot with two stressed syllables (a spondee). Keats maintains the five beats of pentameter, but he shifts one of the beats to create a double stress that mimics the feeling of “slow time.” 

A more complex example appears in line 3. In the version given above, the line features substitutions the first and fourth feet. The first foot is a trochee (stressed–unstressed; “Syl-van”), and the fourth foot is an anapest (unstressed–unstressed–stressed; “who canst thus”). Yet this reading of the line still forces an awkward stress on the final syllable of “historian”:

Syl-van / his-tor- / i-an, / who canst thus / ex-press

However, an alternative reading of the same line is possible, and though it contains only four stressed beats, this version arguably has a more natural cadence in English:

Syl-van his- / tor-i-an, / who canst thus / ex-press

It is perhaps ironic that this line, which is about clarity of expression, involves a metrical ambiguity. This metrical ambiguity may be said to foreshadow the various other forms of ambiguity that will puzzle the speaker and render the urn fundamentally enigmatic.