Base 13

Base-13, tridecimal, or tredecimal is a positional numeral system with thirteen as its base. It uses 13 different digits for representing numbers. Suitable digits for base 13 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, and C, although any 13 characters could be used.

Base 13 in fiction

In the end of The Restaurant at the End of the Universe by Douglas Adams, a possible question to get the answer "forty-two" is presented: "What do you get if you multiply six by nine?" Of course, the answer is deliberately wrong, creating a humorous effect – if the calculation is carried out in base 10. People who were trying to find a deeper meaning in the passage soon noticed that in base 13, 6₁₃ × 9₁₃ is actually 42₁₃ (as 4 × 13 + 2 = 54, i.e. 54 in decimal is equal to 42 expressed in base 13). When confronted with this, the author stated that it was a mere coincidence, famously stating that "I don't write jokes in base 13." See also The Answer to Life, the Universe, and Everything.

Base-13 calendar

The Maya calendar used a base system (the trecena), with 13x20 days for the Tzolkin cycle.