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DISCONTINUED. (Verified 5/2001) RETAINED IN DATABASE FOR REFERENCE. The Zygo Model 21S is a two-choice communication training aid. The system is a set of two lights, one red and one amber, each in its own housing. The lights light up when pressed. The two made be spread apart up to 3 feet or joined directly together. A jack for external switch operation is provided for each light. Additional lights with connecting cable may be ordered. POWER: Battery operated. DIMENSIONS: 3.2 x 6 x 2 inches. WEIGHT: 8 ounces.