Whether a co-occupant's voluntary consent can validate a warrantless search against another occupant who did not consent.
Holding
Yes. Voluntary consent from a person with common authority over, or another sufficient relationship to, the premises or effects can authorize a search against an absent, nonconsenting co-occupant.
Reasoning
The Court treated third-party consent as an established Fourth Amendment principle. Its earlier decisions, particularly Frazier v. Cupp, recognized that a person who jointly uses property may permit its inspection, even when the search produces evidence against another joint user.
Common authority is not a technical question of title or property law. It arises from mutual use of property by persons who generally have joint access or control for most purposes. A co-user may consent in that person's own right, and the other user assumes the risk that a co-user will permit a search of the common area.
Thus, if Graff in fact jointly occupied and used the east bedroom with Matlock, her voluntary consent could make the search valid against him. The Court did not decide the Government's alternative argument that an objectively reasonable but mistaken belief in her authority alone would suffice.