Hexadecimal
Write each character as its code point in base 16.
Hexadecimal is base 16, counting 0 to 9 and then A to F.
It is how character codes are usually written down, because one byte is
always exactly two hexadecimal digits.
That makes the codes two digits wide with nothing between them - a capital A
is 41. Writing always uses capitals, but reading accepts either case, so a
lower case z comes out as 7A and both 7A and 7a read back as z.
The same idea in other bases: binary, decimal and octal.
Examples: