Base 13
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Base 13 is a nonstandard positional numeral system. It may also be called tredecimal or tridecimal. As its names state, 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, X, E and T (similar to base 12) or 0-9, A, B and C (similar to base 16). Base 13 is not used in any practical situation.
Base 13 in fiction
In the end of The Restaurant at the End of the Universe by Douglas Adams, a possible question to have 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, 6 × 9 is actually 42 (as 4 × 13 + 2 = 54). When confronted with this, the author stated that it was a mere coincidence, and that "I may be a pretty sad case, but I don't write jokes in base 13!" See also The Answer to Life, the Universe, and Everything.