The eleventh letter of the Latin alphabet is K. Therefore, three times 11 equals “KKK,” or the Ku Klux Klan.
The number 311 is sometimes used as a greeting to demonstrate membership in the KKK, or simply to express sympathy with the Ku Klux Klan and its ideology.
There is also a long-established rock band from #Nebraska that uses the name “311.” This is not a hate-oriented band and should not be mistaken for one, as it is one of the more historically established alternative rock bands.
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