Base 13

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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, X, E, and T (similar to base 12) or 0-9, A, B, and C (similar to base 16).

[edit] 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). When confronted with this, the author stated that it was a mere coincidence, and that "Nobody writes jokes in base 13 [...] I may be a pretty sad person, but I don't make jokes in base 13." See also The Answer to Life, the Universe, and Everything. Also 42₁₃ is read as "four two base thirteen" as the four is not in a "tens" column.