There are 17 senatorial districts. Each district has two senators who serve staggered four-year terms.
The state's districting system is unique in the United States. Prior to the 2010 Census the state's most populous county, Kanawha County constituted two "superimposed" districts. In practical effect, this meant that Kanawha County was a single district electing two members every two years. The remaining 54 counties of the state were divided into fifteen districts, with county lines not respected in most cases.
Under the unique rule, no multi-county district (and every district except Kanawha's is a multi-county district) may have more than one senator from the same county, no matter the population. This means, for example, that one of the 5th District's two senators must reside in Cabell County and the other must reside in the tiny portion of Wayne County that's inside the 5th District, even though Cabell County has far more people than the portion of Wayne County that is part of the 5th District. However, both senators are elected by everybody within the district, not just by the people of the county in which the senators reside.
Responding to the 2010 Census the Senate redistricted itself. Kanawha County was divided for the first time in the Senate's history, with the northern and western portions joining a part of Putnam County as the 8th District and the remainder of the county constituting the 17th district on it own. This will reduce the number of Senators from Kanawha County from four to three, as one of the 8th's must be a resident of Putnam.
The remainder of the state was redistricted, reflecting the continuing shift of the state's population to the Eastern Panhandle and to Monongalia County, however no incumbents were placed in districts where they will have to run against one another, except for the Kanawha situation. Because senators are elected for four year terms, half of the senate will represent the old districts until the 2014 election.http://www.legis.state.wv.us/legisdocs/2011/1x/maps/senate/Enr%20SB%201006%20Map.pdf