A patent held by a third party that blocks commercial exploitation of a first patent, i.e., an essential patent vis-à-vis the invention in the first patent. The usual solution is for both patent-holders to agree to a cross license, often royalty-free. Frequently, when a company is faced with the risk that a competitor will secure a pioneer patent (because it has, for instance, seen the published application) it will seek to obtain blocking patents on essential improvements.