Limerick
Checker.AABBA, verified

Three rules, strictly enforced by tradition. First: five lines, no more. Second: the rhyme scheme AABBA — the opening pair and the closing line share one sound, while the two short middle lines pair off on another. Third: the shape. Lines one, two, and five run long, around eight or nine syllables; lines three and four run short, around five or six. Long, long, short, short, long.

Underneath the counts sits the anapest, the foot that gives the form its gallop: da-da-DUM, da-da-DUM, da-da-DUM. The long lines carry three of those beats, the short lines two. You can hear it in any limerick you already know — 'There WAS an old MAN with a BEARD'. Break the bounce and the joke dies, however well the lines rhyme.

The fifth line does double duty. It has to rhyme back to the opening sound and it has to land the punchline in the same breath. That is the whole engineering problem of the form, and it is why most failed limericks fail at line five.

Syllables are counted against the CMU pronouncing dictionary, a phonetic database of 134,000 English words, so 'feared' reads as one syllable and 'fired' as two the way a speaker actually says them. Rhyme letters come from the same source: two lines share a letter when their end words share sounds from the last stressed vowel onward. Slant rhymes are accepted, because Lear himself rhymed loosely and your ear forgives more than a strict matcher would.

One honest limit. Anapestic meter — which syllables carry stress — is not something a machine can verify with confidence, since stress shifts with phrasing and emphasis. So the checker verifies the envelope the meter produces instead: line counts, the AABBA pattern, and syllable totals inside the workable range. Pass all three and your limerick has the right skeleton. Whether it gallops is between you and a listener.

The form is older than its name, but Edward Lear made it famous. A Book of Nonsense (1846) collected over a hundred of his limericks, each paired with a scratchy drawing, and turned a pub-and-nursery verse form into a publishing phenomenon. Lear never called them limericks; the name attached itself decades later, possibly from a parlor-song refrain inviting singers to 'come up to Limerick'.

It has always been a performed form. Limericks lived in music halls, barracks, and playgrounds long before anthologies took them in, which is why the sound rules are so rigid while the subject matter famously is not. A sonnet can survive on the page; a limerick is verified out loud.

That is also why the final rhyme has to land squarely. The audience hears line one, holds the sound in memory through three lines of setup, and waits for line five to pay it back. Arrive late, or off-rhyme, and the room notices before any checker does.