ASCII Character Set
The ASCII character set defines 128 characters.
0 to 127 dec. 0 to FF hex. 0 to 177 oct.
ASCII is an acronym for American Standard Code for Information Interchange.
The standard set consists of 128 decimal numbers from zero to 127. These are assigned to letters, numbers, punctuation marks, and special characters.
The first 32 values are non-printing control characters, such as Return and Line feed. Generated by pressing Control and another key. Bell is value 7, Control plus G, shown as ^G.
Notice that Bell decimal value is 7. G decimal value is 71. The Control key subtracts 64 from the value of the key it modifies. So Bell is 64 less than the value of G.
ie 71 - 64 = 7
Here's a jpg with 128 ASCII codes, 0 to 127.