Just as sequences of bits can be used to represent numbers, they can also be used to represent the letters of the alphabet, as well as other characters.
Since all sequences of bits represent numbers, one way to think about representing characters by sequences of bits is to choose a number that corresponds to each character. The most popular correspondence currently is the ASCII character set. ASCII, which stands for the American Standard Code for Information Interchange, uses 7-bit integers to represent characters, using the correspondence shown in table C.5.
Figure C.5: The ASCII Character Set
When the ASCII character set was chosen, some care was taken to organize the way that characters are represented in order to make them easy for a computer to manipulate. For example, all of the letters of the alphabet are arranged in order, so that sorting characters into alphabetical order is the same as sorting in numerical order. In addition, different classes of characters are arranged to have useful relations. For example, to convert the code for a lowercase letter to the code for the same letter in uppercase, simply set the 6th bit of the code to 0 (or subtract 32). ASCII is by no means the only character set to have similar useful properties, but it has emerged as the standard.
The ASCII character set does have some important limitations, however.
One problem is that the character set only defines the representations
of the characters used in written English. This causes problems with
using ASCII to represent other written languages. In particular,
there simply aren't enough bits to represent all the written characters
of languages with a larger number of characters (such as Chinese or
Japanese). Already new character sets which address these problems
(and can be used to represent characters of many languages side
by side) are being proposed, and eventually there will unquestionably
be a shift away from ASCII to a new multilanguage standard
.