Mutual phase locking of Josephson oscillations in a two-junction cell and multijunction arrays is analysed. The locking is due to ac currents of Josephson frequency ω, which flow through some special coupling circuit. For the most important case of almost identical junctions, the locking frequency range is shown to be proportional to the imaginary part of the complex conductivity Y(ω) of the coupling circuit. The power and linewidth of coherent oscillations, as well as the coherence stability with respect to the junction intrinsic noise and parameter spread, have been calculated. The maximum locking range and, hence, the maximum parameter tolerances are shown to take place in the closed-loop-type (ring) structures with long-range junction interactions.