The rhyme scheme of "Sailing to Byzantium" is consistent throughout and the meter is predominantly iambic pentameter. The fact that there may be deviations from strict iambic pentameter is not unusual. Shakespeare did it frequently in his sonnets. It is the dominant meter that counts. Keats' "Ode to a Nightingale" is in iambic pentameter, but consider the line
Fast fading violets covered up in leaves.
The line not only does not scan as iambic pentameter, but it contains eleven syllables. It seems to be intentionally syncopated.
The rhyme scheme "Sailing to Byzantium" is the same in all four stanzas. Take the first stanza:
THAT is no country for old men. The young
In one another's arms, birds in the trees
- Those dying generations - at their song,
The salmon-falls, the mackerel-crowded seas,
Fish, flesh, or fowl, commend all summer long
Whatever is begotten, born, and dies.
Caught in that sensual music all neglect
Monuments of unageing intellect.
The first six lines are ABABAB, and there is a...
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