The eleventh letter of the alphabet is the letter "K"; thus 3 times 11 equals "KKK," or Ku Klux Klan. The number 311 is sometimes used as a greeting to demonstrate membership in the KKK or simply sympathy with the Klan and its ideology.
There is also a longstanding rock band from Nebraska that uses the name "311." This is not a hate-oriented band and should not be mistaken as such.
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