San Francisco is a peninsula connected to Daly City to the south. There's two main throughways that exit SF into DC: 19th Avenue and Mission St.

19th Avenue is closer to the ocean on the Western part of the city. The border's easier to define because the road turns into a freeway once it gets to DC. Mission St. is a little trickier geography-wise as the road continues on through DC. Culturally, the outer SF neighborhood, The Excelsior, hella blurs with DC...so it's really a case of you either know the line or you don't.

I think if you grew up here, you know the distinction. But most of Nu-SF probably haven't traveled past Bernal/Outer Mission, the neighborhoods before the Excelsior.