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The mass splitting between the quarkonium spin-singlet state $h$ ($J^{PC} =1^{+-}$) and the spin average of the quarkonium spin-triplet states $\chi$($J^{PC} = 0^{++}, 1^{++}, 2^{++}$) is seen to be astonishingly small, not onlyin the charmonium and bottomonium cases where the relevant masses have beenmeasured, but in positronium as well. We find, both in nonrelativistic quarkmodels and in NRQCD, that this hyperfine splitting is so small that it can beused as a test of the pure $Q\bar Q$ content of the states. We discuss the $2P$states of charmonium in the vicinity of $3.9$ GeV, where the putative exotics$X(3872)$ and $X(3915)$ have been seen and a new $\chi_{c0}(2P)$ candidate hasbeen observed at Belle.

Quarkonium $h$ States As Arbiters of Exoticity