All transponder codes are four digits, each digit ranging from 0 to 7. This is octal notation—a base-8 number system used for historical and technical reasons in aviation. You will never see a transponder code with an 8 or 9 in it. Examples of valid codes: 1234, 7000, 7700, 7600, 7500. An example of an invalid code: 1289 (contains 8 and 9).

Why octal? Early radars and transponders encoded each digit in three bits of binary code, and three bits represent values 0–7. The system has persisted because it is reliable, and all modern equipment still uses it.

When reading back a code, pronounce each digit separately: "Squawk one-two-three-four" or "Squawk seven-seven-zero-zero."