We show how tailored edge-ray concentrators in two-stage solar collectors that can closely approach the thermodynamic limit to concentration can be constructed with a simple method of strings. The procedure is illustrated for parabolic and Fresnel primary reflectors. The string-construction method is based on the edge-ray principle of nonimaging optics, is a generalization of Fermat＇s principle, and had been developed for the generation of earlier ideal nonimaging concentrators. The string method can supplant the more time-consuming procedure of numerical solution of the reflector governing differential equation.