Carols started out as a form of singing circular folk
dance. Plato inveighed against their ancestors, so there must be something good about them.
Carols do vary widely in form, especially these days, but back around the 15th century, they were more standardized.
A carol started out with a "burden" or short chorus. This was usually a rhymed couplet. That was followed by verses that were often short-lined quatrains
with rhyme of bbba. The last lines of the quatrains were sometimes a foot or so shorter than the other three. The burden might appear as a chorus between
verses, or it might be used as two refrains that appeared as the last line of the verses. There were many variations, but this covers the