Given all the recent hubbub about pentaquarks, it might
be worth backing up a step and talking about plain old
diquarks and triquarks.
For starters, let's restrict attention to the three "light"
quarks: up, down, and strange. These are an order of magnitude
less massive than the other quarks, so this is a reasonable
dichotomy.
Mesons are by definition composed of a quark and an antiquark.
Obviously there are nine such mesons: there are three ways
of picking the quark, and three ways of picking the antiquark.
So, the question arises: Why have people been talking for
50 years about the "eightfold" way whereby "eight" mesons
form an "octet"????
Let's be clear: The three lightweight quarks combine to
form *nine* mesons.
It's a bit of work to understand how the quarks combine to
form the mesons, but it can all be understood in analogy
to things you already know. It's even possible to pick
out 8 of the 9 and call them "the" octet, although I'm not
convinced this is worth the trouble.