What is contributory infringement?

Within the context of patent law, contributory infringement is a type of indirect infringement. Specifically, contributory infringement occurs where someone assists another in their efforts to infringe a patent by selling, offering to sell or importing one or more components which can be used to make the invention which is protected by a patent. However, the parts sold must have no other substantial use except for being used in the patented invention - if they do have other substantial uses, there is no contributory infringement. For example, suppose you have a patent for a pencil with an eraser, and I start selling eraser nubs. If you can show that those eraser nubs serve no purpose except for being put on the top of a pencil, I am a contributory infringer.