The doctrine of indivisibility (or indivisibility doctrine) was a legal doctrine in United States copyright law, which held that a copyright was a single, indivisible right that its owner could only assign as a whole. The doctrine was founded upon the policy concern that a defendant alleged to have infringed a single work might find himself facing claims from multiple plaintiffs, all claiming copyright in that same work. Despite the indivisibility doctrine, a copyright holder could still effectively assign certain rights. The assignees of those rights were held to be "mere licensees."