For many decades, San Mateo County, California, has been the only county in California that elects its county supervisors at-large. Even San Mateo County had five supervisor districts, but the only purpose of the supervisor districts was to make it possible for a residency requirement to exist. The voters in the entire county chose each supervisor, but the law required that each seat was numbered, and assigned to a particular district. So only residents of one particular area could run for any particular seat.

On November 6, San Mateo County voters voted for Measure B, to elect supervisors in future elections on a district basis. Of course, the measure could not take effect until the 2014 election. At the November 2012 election, the winner for the district four seat, Warren Slocum, actually lost to Shelly Masur within the boundaries of district four. But, under the old system, which was still in effect, that made no difference. The vote throughout the entire county favored Slocum over Masur, so Slocum is elected. Ironically, Slocum favored Measure B, even though if it had been passed two years ago, he would have lost. See this story.