Complex sentence: A Fourteener, in poetry, is a line consisting of 14 syllables, usually having 7 iambic heptametric feet, most commonly found in English poetry produced in the 16th and 17th centuries.
Split into simpler sentences: A Fourteener, in poetry, is a line consisting of 14 syllables, known as iambic heptameter when it contains 7 iambic feet, as usual. It is most commonly found in English poetry produced in the 16th and 17th centuries.