This study examined the stability and variability of interpersonal coordination, in which one person breathed while the other moved a wrist back and forth with an inverted pendulum in hand. Nine pairs of subjects coordinated each other's movement in two relative phase modes. In one mode, Radial flexion-Inspiration and Ulnar flexion-Expiration (RIUE), one subject radially flexed the wrist as the other inhaled and ulnarly flexed it as the other exhaled. In the other, Ulnar flexion-Inspiration and Radial flexion-Expiration (UIRE) mode, the wrist was ulnarly flexed at inhalation, and radially flexed at exhalation. Results were as follows: (1) The two were more highly coordinated in RIUE mode than UIRE mode as the frequency of oscillation increased. (2) Phase transitions were observed from URIE to RIUE mode, as the frequency of oscillation increased. And (3) the more different in preferred frequency the pendulum and breathing movements were, the more deviated from the intended relative phase the coordination became. These results suggest that interpersonal coordination of breathing and wrist-pendulum movement is qualitatively equivalent to intra personal coordination between them.