Limericks must contain exactly 39 syllables arranged in a pattern across five lines. The line-by-line syllable pattern is 9-9-6-6-9, and the rhyming pattern is a-a-b-b-a.
The third and fourth lines in a limerick are usually shorter than the first, second and fifth lines. The intent of a limerick is frequently to be humorous with regard to sometimes obscene subject matter. A person and a place are usually introduced in the first line, and geographic names are often exploited, particularly exotic ones. The final line of a limerick reflects some sort of a conclusion that applies to the person introduced in the first line. Edward Lear, who wrote 212 limericks, helped to popularize the form in his "Book of Nonsense" published in 1845.