SAN FRANCISCO — Siding with a plaintiff in a major arbitration-related appeal, the California Supreme Court on Thursday ruled that an arbitrator, rather than a court, should determine whether a California car dealership’s arbitration clause allows employees to arbitrate on a classwide basis.

Writing on behalf of a 4-3 majority, Justice Kathryn Mickle Werdegar found that no “universal rule” allocates who should make the decision about whether class arbitration is available. California state law establishes no presumption that courts should decide and federal arbitration law doesn’t force a contrary result, Werdegar wrote. Ultimately, she concluded, the decision comes down to the parties’ underlying agreement.