Counting form factors of twist-two operators

We present a simple method to count the number of hadronic form factors based on the partial wave formalism and crossing symmetry. In particular, we show that the number of independent nucleon form factors of spin-$n,$ twist-2 operators (the vector current and energy-momentum tensor being special examples) is $n+1.\mathrm{}$ These generalized form factors define the generalized (off-forward) parton distributions that have been studied extensively in the recent literature. In proving this result, we also show how the $J^{\mathrm{PC}}$ rules for quarkonium states arise in the helicity formalism.