Because it has a reciprocal of unique period length among primes, 11 is a unique prime. 11 goes into 99 exactly 9 times, so fractions with 11 in the denominator have two digit repeating sequences in their decimal expansions. Multiples of eleven by one-digit numbers all have double digits: 00 (=0), 11, 22, 33, 44 . . .
In base 10, there is a simple test to determine if an integer is divisible by 11: take every other digit and add them up, then take the remaining digits and add them up. If the difference between the two sums is a multiple of 11, including 0, then the number is divisible by 11.
In base thirteen and higher bases (such as hexadecimal), eleven is represented as B, where ten is A. In duodecimal, however, eleven is sometimes represented as E and ten as T.
There are eleven players in a soccer as well as in a cricket team.
The number eleven is important in numerology, as it is the first of the master numbers.
Being only one hour before 12:00 -- midnight -- the eleventh hour means the last possible moment to take care of something, and often implies a situation of urgent danger or emergency (see Doomsday clock).
Uri Geller has always been fascinated by the number eleven. After the September 11 World Trade Center attack, he published an open list of connections to the number eleven involved in that attack.
In the film This Is Spinal Tap, guitarist Nigel Tufnel proudly shows off an amplifier with top settings of 11, higher than the traditional 10.
