I don’t know if this is common among Latino gangs or if it’s just in L.A., but if the name of the gang is based on the neighborhood or district, they append the numeral ‘13’ to the name. In graffiti, this appears variously as ‘XIII’, ‘X3’, ‘13’. I know they’ve been doing this for at least 30 years.
When I lived in L.A. I knew some Hispanic gang members and I asked them this same question because the ‘13’ or ‘XIII’ is ubiquitous in graffiti around there. Believe it or not, the answer I got was that M is the 13th letter of the alphabet and it stood for Marijuana. In other words, they told me the ‘13’ meant that they smoked pot. I’m not making this up, but I wouldn’t blame you if you took their answer with a grain of salt.
To follow up on Bob Scene’s reply, I initially thought it might have been a borrowing from '60s biker parlance wherein 13 was a reference, as noted, to the letter “M” and meant the bearer (in the case of biker colors so adorned) was a marijuana smoker.
I Live in Santa Ana, California and I can tell you that ‘13’ means Sureno gangs, or Southern California gangs where as ‘14’ means Norteno gangs. IIRC , the 13th letter in the Spanish alphabet was M, which stood for Maravilla, one of the original gangs ( I’m not 100% sure, correct me if I’m wrong here.) I’m positive that 13 means Sureno and 14 means Norteno, though. Around here there is a 17th Street gang [ XVII] and in LA the 18th Street Gang is pretty well known.
I’ll agree with what Kaps has said. (Nortenos = 14, Sureno = 13). Not only is this evidenced by seeing more 14 tagging in Northern CA, but this is what was taught my sister in a police course for high school teachers. (and please do not assume the cops don’t know what’s going on.)
That said, it’s entirely possible that it could change, or is changing, since not all the gangs are aware of the history or origins of the names, so it may take on a new meaning in different areas. (not every city has a 13th or 14th St, but in those that do, it could have become the symbol of the street.)