Mesons: hadrons with integer spin, usually quark-antiquark states I came across this statement in a pdf of a powerpoint. Unfortunately the presentation is so badly formatted (missing images, content overlapping content, etc.) that it's hard to follow and I can't see if he ever describes mesons that are not quark-antiquark states. Are there mesons that aren't quark-antiquark pairs? The only definition I've ever seen is: Mesons are hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. The only thing I can think of is that he might classify tetraquarks as mesons because of their integer spin?