Seven's Stubborn Role

Among the single digits, seven is the odd one out. It is not a factor of 10, so its multiples never land on round numbers the way 2s, 5s, and 10s do. Seven is also prime, meaning its only positive factors are 1 and 7 — and primes are the atoms of arithmetic, the numbers everything else is built from.

That same stubbornness shows up all over life: seven days in a week, seven notes in a musical scale, seven colors named in a rainbow. The number that refuses to divide neatly in math also resists tidy grouping in culture.