Tetradics transform Dyadics in much the same way that Dyadics transform Vectors. They are represented using Hebrew characters and have 81 components (Morse and Feshbach 1953, pp. 72-73). The use of tetradics is archaic, since Tensors perform the same function but are notationally simpler.
References
Morse, P. M. and Feshbach, H. Methods of Theoretical Physics, Vol. 1. New York: McGraw-Hill, 1953.