aabba (11. 594)

Via Futility Closet, we are reminded of the anatomy of a limerick (with the above rhyme scheme, see previously) with the following meta-versification by John Irwin, poet and professor of the humanities: 

A limerick’s cleverly versed—
The second line rhymes with the first;
The third one is short,
The fourth’s the same sort,
And the last line is often the worst.

This rendition is almost certainly in homage to the anonymous exemplar: 

The limerick packs laughs anatomical
Into space that is quite economical.
But the good ones I've seen
So seldom are clean
And the clean ones so seldom are comical.

Heretofore, most often privileged showmanship, they often invoked exotic geographic locations as a way to subvert the rote teaching of the subject in schools, with several variations and violations. British wordplay and maths expert Leigh Mercer, best known for his palindromes, (¡“A man, a plan, a canal: Panama!), also famously a mathematical limerick:  

12 + 144+20 +3 ✖️ √4 / 7 + (5 ✖️ 11) = 9² + 0 

Or, as read: 

A dozen, a gross, and a score
Plus three times the square root of four
Divided by seven
Plus five times eleven
Is nine squared and not a bit more.