A regular pattern of unstressed and stressed syllables in a line or lines of poetry. Below is an illustration of some commonly used metrical patterns:
Iamb (unstress, stress): Whose woods / these are / I think / I know
Trochee (stress, unstress): Irish poets / learn your / trade
Anapest (unstress, unstress, stress): As I came / to the edge / of the woods
Dactyl (stress, unstress, unstress): Half a league, half a league, half a league, onward
Spondee (stress, stress): Now, by / heaven ;