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The longitudinal mode spectrum of a solid-state ion laser can be greatly simplified and stabilized by spacing the rod and the mirrors according to a simple rule: the optical path between each end mirror and the nearest end of the rod should be an integral multiple of the optical length of the rod. Application of this rule can reduce the number, increase and regularize the frequency spacing, symmetrically distribute the intensities, and increase the temporal stabilization of the oscillating longitudinal modes. These remarkable effects were demonstrated with a continuous Nd:YAG laser that was constructed with several different spacings of cavity elements. The rule is probably the result of absorption loss that is localized at the surfaces of the rod. Other models, limitations, and generalizations of the rule are also discussed.