Choriamb
In Greek and Latin poetry, a choriamb is a metron (prosodic foot) consisting of four syllables in the pattern long-short-short-long (— —), that is, a trochee alternating with an iamb. Choriambs are one of the two basic metra that do not occur in spoken verse, as distinguished from true lyric or sung verse. The choriamb is sometimes regarded as the "nucleus" of Aeolic verse, because the pattern long-short-short-long pattern occurs, but to label this a "choriamb" is potentially misleading. In the prosody of English and other modern European languages, "choriamb" is sometimes used to describe  four-syllable sequence of the pattern stressed-unstressed-unstressed-stressed (again, a trochee followed by an iamb): for example, "over the hill", "under the bridge", and "what a mistake!". English prosody. In English, the choriamb is often found in the first four syllables of iambic pentameter verses, as here in Keats' "To Autumn":