impossible to gain an absolute and complete knowledge of every publication on earth, not to mention unpublished patent applications, there is always some degree of uncertainty. If the independent claim is determined to be invalid, however, a dependent claim may nevertheless survive, and may still be broad enough to bar competitors from valuable commercial territory. Under the European Patent Convention, when a claim in one particular category (see below), e.g. a process claim, depends on a claim from a different category, e.g. a product claim, it is not considered to be a dependent claim but an independent claim. Under U.S. law, this is still counted as a dependent claim, regardless of the class change. The rules of claim drafting also permit "multiple dependent claims" that reference more than one other claim, e.g.: "3. Method of claim 1 or 2, further comprising..." The rules for this are quite specific: specific claims must be referenced ("the method of any of these other claims" is incorrect); the claims must be referenced in the alternative ("the method of claims 1 and 2" is incorrect); etc. While still acceptable, this claim style is seldom used in the U.S. because it is counted for filing fee purposes according to the number of claims that it references. Thus, if the claim depends from three former claims, it is counted for fee purposes as three dependent claims. In light of the "excess claim" fees currently imposed by the USPTO, this tactic can quickly become expensive. Multiple dependent claims are, however, very commonly used in other jurisdictions, including Europe. Claims can also be classified in categories, i.e. in terms of what they claim. A claim can refer to a physical entity, i.e. a product (or material) or an apparatus (or device, system, article, ...). The claim is then called respectively "product claim" or "apparatus claim"; or an activity, i.e. a process (or method) or a use. The claim is then called respectively "process claim" (or method claim) or "use claim". In the United States, these categories are called the four statutory categories of invention and are called processes, machines, manufactures, and compositions of matter. Special types of claims In addition to the above basic claim types, there are also many special types of claim which are used in different circumstances. Sometimes a particular claim form is required by law if a patent is to be granted for a particular