th 1 as the leftmost (highest-weight or seventh) bit. We also observe that all upper-case letters have 0 in the second leftmost (or sixth) bit position, while lower-case letters have 1 instead. Thus, the 2 X 26 = 52 upper and lower-case letter alphabet is actually using six-bit codewords, while the 26-letter alphabet is using five-bit codewords. A five-bit codeword can cover 25 = 32 characters, which is apparently not sufficient for representing the 26 letters and the 10 numbers, unless numbers can be written as words (e.g. 3 = three ). To increase the possibilities of a five-bit code, a trick is to introduce two shift characters, one to announce a shift from letters to figures (concerning codewords to follow) and the other for the reverse operation. The introduction of these two shift characters, thus, virtually permits one to re-use the set of 26 letter codewords as another set of 26 figure codewords, including numbers and punctuation, leaving four extra symbols to use for space, carriage return, line feed, and blank. This is the principle 4 For a detailed introduction to early secret codes and cryptography, see for instance E. Desurvire, Wiley Survival Guide in Global Telecommunications, Broadband Access, Optical Components and Networks, and Cryptography, (New York: J. Wiley & Sons, 2004). 5 American Standard Code for Information Interchange. 6 Extended Binary Coded Decimal Interchange Code. 7 See, for instance, http://-wikipedia.org/wiki/ASCII. 8.2 Coding language Table 8.1 ASCII code table for common keyboard characters (extract). Bit word space of the Baudot code, now better known as International Alphabet IA2 and still in use in telex machines. Consider now the code-source entropy. If all ASCII codewords were equally likely (having a uniform probability distribution), the corresponding entropy would be H = log 128 = log 27 II 7 bit/symbol, which is precisely the codeword length. In the case where the codeword length matches the entropy of the code source, it is said that the code is optimal. This important