Diagram of mass versus spin period for 39 NSs. The horizontal line M = 1   M_⊙ (3.2 M_⊙) stands for the measured minimum mass (theoretical maximum mass, see Rhoades & Ruffini 1974). The vertical line at 20 ms separates the samples into two groups, MSP (<20 ms) and less recycled NS (>20 ms). It is found that the mass averages of two groups are, respectively, 1.57 ± 0.35   M_⊙ and 1.37 ± 0.23   M_⊙. The solid curve stands for the relation between accretion mass and spin period of recycled pulsar as described in Eqs. (1) and (2),  (M_⊙).