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Most digital circuits use a binary system with two voltage levels labeled " 0 " and " 1 ".

Often logic " 0 " will be a lower voltage and referred to as " Low " while logic " 1 " is referred to as " High ".

However, some systems use the reverse definition (" 0 " is " High ") or are current based.

Ternary ( with three states ) logic has been studied, and some prototype computers made.