Washington state constitution
Article IV
Section 2

The supreme court shall consist of five judges, a majority of whom shall be necessary to form a quorum, and pronounce a decision. The said court shall always be open for the transaction of business except on nonjudicial days. In the determination of causes all decisions of the court shall be given in writing and the grounds of the decision shall be stated. The legislature may increase the number of judges of the supreme court from time to time and may provide for separate departments of said court.