What is a metrical line?
In poetry, the meter (American English) or metre (British English) is the basic rhythmic structure of a verse or lines in verse. Many traditional verse forms prescribe a specific verse meter, or a certain set of meters alternating in a particular order. The study of metres and forms of versification is known as prosody. (Within linguistics, "prosody" is used in a more general sense that includes not only poetical meter but also the rhythmic aspects of prose, whether formal or informal, which vary from language to language, and sometimes between poetic traditions.)