[chukaefu] Limericks are 4 line poems

Two rhyming lines, followed by a line with an internal rhyme (usually written as two lines), followed by a line recapitulating the original rhyme of first two lines.

Drop the internal rhyme in the third line and it still sounds limericky.

Thinking about it as four lines is useful if you wish to set a limerick to music, which is usually composed in phrases of four, not five.