Four Humors

The words sanguine, choleric, melancholy and phlegmatic are all equally uncommon. Not so obscure that you don't know them, and definitely not so common that you hear them every day, but all a *little* obscure. Plus, they all have different, common, English adjectivifying suffixes. They complement one another extremely well. When you come to think of it, it's pretty unlikely that it should have turned out that way.

Bonus: Bilious too. AWESOME!