| Binary (base 2) | Binary is base-2: every digit (bit) is 0 or 1. It is how computers store and process all data at the hardware level. |
| Octal (base 8) | Octal is base-8, using digits 0–7. Each octal digit maps to exactly three bits, which is why it appears in Unix file-permission modes such as 755. |
| Decimal (base 10) | Decimal is base-10, the everyday number system using digits 0–9. |
| Hexadecimal (base 16) | Hexadecimal is base-16, using 0–9 then A–F. Each hex digit is exactly four bits, so it is a compact way to write bytes — used for colours (#FF8800), memory addresses and MAC addresses. |