Abstract: A combination system such as may, for example, be embodied in a matching game or instructional device or in a set of locks and keys consists of a number of receivers and an associated number of plugs with both the receivers and the plugs having cooperable coding features coded according to unique rules, derivable from N-dimensional geometry, assuring a desired form of cooperation between a plug inserted in a receiver only when the code of such plug matches the code of the receiver.