San Francisco, California has both major, well-known neighborhoods and districts as well as smaller, specific subsections and developments. While there is considerable fluidity among the sources, one guidebook identifies five major districts, corresponding to the four quadrants plus a south central district. These five broad districts, counterclockwise are: Central/downtown, Richmond, Sunset, Upper Market & beyond (south central) and Bernal Heights/Bayview & beyond (southeast). Within each of these five districts are located major neighborhoods, and again there is considerable fluidity seen in the sources. The San Francisco Planning Department officially identifies 36 neighborhoods. A group of researchers at Theory.org did a study of classified advertising of housing rentals to extract neighborhood names in the vernacular, and identified 40 neighborhood names in common use. Within these 36 official neighborhoods are a large number of minor districts, some of which are historical, and some of which are overlapping.