California's South-Side 13 gang members wearing blue bandana masks and sunglasses, posing for the camera making gang sign at the end of the Mexican Day Parade in New York City.
Southside colors are blue. Thirteen stands for M, the thirteenth letter of the alphabet. M is for Mexican Mafia.
MS13, S13, M13, are commonly worn on gang clothing or jerseys, written Old English style. The gang territory in California that extends from North of Monterey to the Mexico border.