A systematic variation of shape variables $\beta$ and $\gamma$ with $N$ and $N_{p}N_{n}$ is studied in the framework of an asymmetric rotor model of Davydov and Filippov for the Z =50–82, N =82–126 major shell space. The role of the Z = 64 subshell in producing smooth systematics has been discussed. The quadrupole moments are extracted after considering both axially symmetric and axially asymmetric nuclei. The correlation of $\beta$ with $\gamma$ together with the measured quadrupole moments indicates that $\gamma$ -rigidity is better observed in nuclei with modest deformation.
