The below table visualizes how the decimal number 21 equals the octal number 25.
Octal numbers are a positional numeral system with the base (or "radix") 8, which means it consists of the digits 0, 1, 2, 3, 4, 5, 6 och 7. In the octal system, the number 8 is written as 10.
Since 8 is equal to 23, each octal digit can be used to describe three binary digits. This was frequently used early on in the computer era, when the word length in computer processors was often a multiple of 3, such as 6, 12 or 24 bits. In modern computers the word length is usually a multiple of 8, such as 32 or 64 bits, and therefore hexadecimal numbers are better suited to describe data nowadays.