Digit One

1 (One) is ASCII code point 49 (0x31), Unicode U+0031, and a printable character in the Digit category. It is the multiplicative identity — the only number you can multiply by without changing anything — and the smallest positive integer, the starting point for counting in nearly every human culture. In code, 1 is the universal true. C had no boolean type before C99, so 1 stands in for truth everywhere, and comparison operators return it by convention. It is the seed of off-by-one errors, the most common bug genus in programming: fence-post problems, `<=` vs `<`, forgetting that `substring(0, n)` excludes index n. In bitwise operations, `n & 1` is the idiomatic way to test odd/even, cleaner than `n % 2`. Lua and MATLAB index arrays from 1 instead of 0, which means every developer crossing that border will misplace an element at least once. The vertical stroke descends from the Brahmi numeral, a simple horizontal bar. The upward serif at the top appeared in medieval European manuscripts to distinguish it from lowercase l and uppercase I — a problem that persists in sans-serif fonts to this day. UTF-8 encodes it as the single byte 0x31, one position above 0x30 in the contiguous ASCII digit range, so the byte-to-value subtraction trick works just as naturally. Unicode names it DIGIT ONE under category Nd. Watch for lookalikes: the fullwidth １ (U+FF11) appears in East Asian text, and the superscript ¹ (U+00B9) won't match `\d` in most regex engines.