In aviation, the aircraft's transponders transmit a code so that they can identify one another. This code uses the octal system. The octal system uses the base-8 number system, with values made up of the digits 0 to 7. This is in contrast to the decimal system (which we are used to working with), which uses base-10 and digits 0 to 9. In base-10, each digit is a power of 10. For example, the number 125 is calculated as: