Abstract: A method of error detection in a digital system in which at least one digit in every n (where n is an integer greater than two) is omitted from use as the "particular digit" in the comparison of succeeding particular digits in a received pattern with the combination in accordance with a predetermined function of at least two digits preceding the particular digit in question by a particular spacing. The pattern is a pseudo-random binary sequence of maximal length and every third digit is taken as the particular digit. Every digit forms part of a comparison whether as "particular digit" or as one of said "at least two digits preceding". Shift registers, means to divide digits between the shift registers, and modulo -2 adders are used to carry out the method.