For those of you who rant and curse
When faced with rhymes, they could be worse.
Mind you, if you're reading a villanelle you'll be faced with five tercets in a row followed by a four-line verse (a quatrain) which makes nineteen lines, which is even longer than a sonnet.
(Speaking of which, tercets are quite often hidden cunningly inside in longer verses, and some sonnets end with a couple of them.)
The most famous and celebrated tercets are those of Dante, who wrote a whole book full of them. He used a particular rhyme scheme called terza rima, where the rhymes of the first tercet go ABA, then the next BCB, and so on. It works brilliantly in Italian, but is surprisingly difficult to translate into English.
The other problem with terza rima is that the link between the verses means that it never really reaches an ending, so usually you have to have a single line to finish things off.
Still, tercet is a lovely, elegant little word, and I personally could easily cope with more of them.
Nuts and Bolts: tercet. This word comes from the Italian terzetto, which means little third.