  The aim of this paper is to present necessary and sufficient conditions for
generalized H\"{o}lder's inequality on generalized Morrey spaces. We also
obtain similar results on weak Morrey spaces and on generalized weak Morrey
spaces. The necessary and sufficient conditions for the generalized
H\"{o}lder's inequality on these spaces are obtained through estimates for
characteristic functions of balls in $\mathbb{R}^d$.
