If one or both resonator mirrors exhibit different radii of curvature in the x- and the y- directions (cylinder mirrors or toricResonatorstoric mirrorsMirrortoric), the g-parameters of the resonator, and consequently the resonator properties, become different for the two directions. Different mode properties in the x and the y directions are also obtained for spherical resonator mirrors if the corresponding effective resonator lengthsEffective resonator length are different. This happens if the surface normals of intra-cavity optical elements exhibit an angle with respect to the optical axisOptical axis. Examples are solid state slab lasersSlab lasersolid state and ring resonatorsResonatorsunstable. In most of these cases, the resonator works stable or unstable in both directions and the output beam can be described by astigmatic stable or unstable resonator modesPropagationof unstable resonator modes.