for the purpose of calculating the checksum, the top and bottom halves of each character can be assigned the values shown in the table below. each such value is derived by assigning weights of 4,2,1 and 0 to the extensions according to their position in the character, summing the weights, and taking modulo 6 of the sum. for example the symbol for 'b' has bottom half extensions of its first two bars, represented below as 1100, the sum of their weights being 4+2+0+0 = 6, modulo 6 of which is 0.