Most limericks have a rhyme scheme like this one: a/a/b/b/a.

The meter is mostly iambic tetrameter, except for the two bb lines, which is short a beat or two. Again, this is a typical pattern in limericks. Usually, the two rhyming lines in the middle of a limerick have one beat less - which would make it trimeter -- but this is not always the case. It is always different, but the rhythm does not always go from tetrameter to trimeter. Also, limericks are usually in iambic meter - unstressed followed by stressed.