Status of exotic-quantum-number mesons

The search for mesons with non-quark-antiquark (exotic) quantum numbers has gone on for nearly 30 years. Currently there is experimental evidence of three isospin one states, the π1(1400), the π1(1600), and the π1(2015). For all of these states, there are questions about their identification and even if some of them exist. In this article, we will review both the theoretical work and the experimental evidence associated with these exotic quantum number states. We find that the π1(1600) could be the lightest exotic quantum number hybrid meson, but observations of other members of the nonet would be useful