Do you capitalize the initial "S" of "southern" in "southern california"?
This is a tricky one. Most formal guidelines I've seen would indicate that you should not capitalize "southern" in this context because it it merely refers to part of state based on the point of a compass. Southern California is not a formal or political entity, it is simply a description of a geographic portion of one.

Still, you will often see it capitalized by local English speakers. The area's most prominent newspaper, the Los Angeles Times, even does it. In reference to other states, "southern" is never capitalized (as in southern Nevada, or southern Georgia), so what makes the southern half of California special? The reason for this would seem to be that many consider "Southern California" to be the proper name of a clear-cut and well-known region, as is "Northern California". Given its unique geography (large, long and thin) and two distinct cultural nodes (Los Angeles and San Francisco), California is easily divided into two regions. The frequent use of these names among the public to describe the two regions has caused them to develop beyond generic descriptions and into proper names according to this line of thinking.

So, the formal answer is that "southern " should not be capitalized, but the more pragmatic answer is that, depending on meaning, it can be.
In fact, the Associated Press Stylebook (yes, it's one word) recognizes "Southern California" as an established region, with a capital S, putting it in the same category as the South Side of Chicago or the Lower East Side of New York.

I don't have my Chicago Manual of Style any longer; it would be interesting to read their take on this.
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You can do that if you like. It's up to you. California has to be capitalized.

California should be capitalized. "Southern" is just a regular adjective, which doesn't need capitalization.
But it's important to note that names of places such as the following must be capitalzied:
South Pole
South Carolina
West Germany
Easter Island
West Hollywood
 YoungCalifornian's reply was promoted to an answer.
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I think the only hint possibly linking the capitalization of "southern" to California is that there was a time certain political force was trying to divide California into two separate states, much like North and South Carolina. You are right about the usage of "SouthernCalifornia". Perhpas The L.A. Times still has few remnants of that movement working there.Emotion: big smile
 BarbaraPA's reply was promoted to an answer.
Formal rules and customs are important, but so is the appearance of construction. In the context of California, I always capitalize Southern or SoCal because otherwise it looks awkward. And that separatist movement for 2 California's is still alive in certain circles!
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