Decimal
Write each character as its code point in base 10.
Every character a computer stores has a number, and until Unicode came along those numbers ran from 0 to 255. This writes each character as that number in ordinary base 10.
255 needs three digits, so every code is padded to three - a capital A is
065, not 65. That fixed width is what lets the codes run together with
nothing between them and still be read back one at a time. Binary,
hexadecimal and octal do the same thing in
base 2, 16 and 8.
If you are after A = 1 rather than A = 65, that is
letter numbers instead.
Examples: