Washington Constitution Art. 2, § 6. Election and Term of Office of Senators

After the first election the senators shall be elected by single districts of convenient and contiguous territory, at the same time and in the same manner as members of the house of representatives are required to be elected; and no representative district shall be divided in the formation of a senatorial district. They shall be elected for the term of four years, one-half of their number retiring every two years. The senatorial districts shall be numbered consecutively, and the senators chosen at the first election had by virtue of this Constitution, in odd numbered districts, shall go out of office at the end of the first year; and the senators, elected in the even numbered districts, shall go out of office at the end of the third year.