The word "comprising" in a claim renders the claim open, which means that additional elements could be added to the accused infringing device without avoiding infringement. For example, if a claim stated that a chair had three legs and employed the word "comprising," a chair having four legs would infringe. At the other end of the spectrum is a closed claim, which uses the expression "consisting of. " This means that in a product alleged to infringe, if meaningful additional elements are added to the features recited in a claim there is no infringement. These types of claims frequently appear in the chemical and metallurgical arts. There is an intermediate scope of claim that uses the expression "consisting essentially of" and means that some additional components may be employed in the allegedly infringing product without avoiding infringement.