The public domain is the realm of material—ideas, images, sounds, discoveries, facts, texts—that is unprotected by intellectual property rights and free for all to use or build upon. Our economy, culture and technology depend on a delicate balance between that which is, and is not, protected by exclusive intellectual property rights. Both the incentives provided by intellectual property and the freedom provided by the public domain are crucial to the balance. But most contemporary attention has gone to the realm of the protected.