Despite the pseudoscalar mesons' masses being known to high precision, and being the most well studied and understood mesons, the decay properties of the pseudoscalar mesons, particularly of eta (η) and eta-prime (η ′), are somewhat contradictory to their mass hierarchy: While the η ′ meson is much more massive than the η meson, the η meson is thought to contain a larger component of the relatively heavy strange and anti-strange quarks, than the η ′ meson does, which appears contradictory. This failure of the quark model to explain this mass difference is called the " η-η ′ puzzle ".