The content-independent school of limerickery holds that any five-line poem with the requisite structure is a limerick, as would be true for a sonnet or villanelle fitting their respective formulae.
Limericks are officially described as a form of 'anapestic trimeter'; the 'anapest' is a 'foot' of poetic verse consisting of three syllables, the third longer (or accentuated to a greater degree) than the first two. Lines one, two and five of a limerick should ideally consist of three anapests each, concluding with an identical or similar phoneme to create the rhyme. Lines three and four are shorter, constructed of two anapests each and again rhyming with each other. Thus, the overall rhyme structure of a, a, b, b, a, with the beat pattern
a:da-da-daah da-da-daah da-da-daah
Often, lines three and four have an extra syllable at their start. Variations on this theme include the substitution of the final foot of a line to the iamb, a two-syllable foot with the accent on the second. Further substitution in this way can result in the maximum syllable count of
1. 9 syllables pause 3 1. da-da-daah da-da-daah da-da-daah
2. 9 syllables pause 3 2. da-da-daah da-da-daah da-da-daah
3. 6/7 syllables no pause 3. (da) da-da-daah da-da-daah
4. 6/7 syllables no pause 4. (da) da-da-daah da-da-daah
5. 9 syllables pause 3 5. da-da-daah da-da-daah da-da-daah
being reduced to a minimum of
1.7 syllables pause 5 1. da-dah da-da-dah da-daah
2. 7 syllables pause 5 2. da-dah da-da-dah da-daah
3. 4 syllables pause 2 3. da-da da-daah
4. 4 syllables pause 2 4. da-da da-daah
5. 7 syllables pause 5 5. da-dah da-da-dah da-daa
As the figures in italics indicate, curtailing the 'active' beats of any line results in a corresponding increase in the number of beats' pause between lines.
It is possible to construct a limerick with unmatching a or b lines; it is essential that the overall beat structure remains and that the flow of words allows the lines to be spoken as if they were identical.