Characters partitioned into sequential groups

• control characters (0..31) ... e.g. '\0', '\n'
• punctuation chars (32..47,91..96,123..126)
• digits (48..57) ... '0'..'9'
• upper case alphabetic (65..90) ... 'A'..'Z'
• lower case alphabetic (97..122) ... 'a'..'z'

Sequential nature of groups allows for things like (ch - '0') Eg. '4' - '0' converts the character '4' into the integer 4.

See   man 7 ascii