In a patent or patent application, the claims define, in technical terms, the extent, i.e. the scope, of the protection conferred by a patent, or the protection sought in a patent application. In other words, the purpose of the claims is to define which subject-matter is protected by the patent (or sought to be protected by the patent application). This is termed as the "notice function" of a patent claim´to warn others of what they must not do if they are to avoid infringement liability. The claims are of the utmost importance both during prosecution and litigation alike.
In most jurisdictions, a patent is a right to exclude others from making, using, importing, selling or offering for sale the subject matter defined by the claims when the claim is for a thing (apparatus, composition of matter, system, etc.). If a claim is for a method, the right to exclude would be to exclude any single party from carrying out all the steps of the claim. In order to exclude someone from using a patented invention, the patent owner, or patentee, needs to demonstrate in a court proceeding that what the other person is using falls within the scope of a claim of the patent; therefore, it is more valuable to obtain claims that include the minimal set of limitations that differentiate an invention over what came before (i.e., the so-called prior art). But the fewer the limitations in a claim, the more likely it is that the claim will cover, or "read on," what came before and be rejected during examination or found to be invalid at a later time for lack of novelty or obviousness.
No patent system today is a purely either central or peripheral, but the system used in Germany and most of the other countries of continental Europe is considered more central than the system currently used in the United Kingdom, the United States, and especially Japan, which are more peripheral. In recent years,[vague] Japan's system has become more peripheral, while the system used in the United States was becoming more central because of the increasing resort in US patent litigation to the doctrine of equivalents to expand claim scope unpredictably, until the US Supreme Court reversed that trend in the Warner-Jenkinson case in 1997.
Regarding the structure of a claim, under the EPC, what is called the "preamble" is different from the meaning the "preamble" has under U.S. patent law. In an independent claim in Europe, the preamble is everything which precedes the expression "characterized in that" or "characterized by" in a claim written according to the so-called "two-part form", and therefore everything which is regarded as known in combination within one prior art document, namely the closest prior art document. For this reason, in Europe, the preamble of a claim is sometimes also called "pre-characterizing portion".
The claims often use precise language. Certain words commonly used in claims have specific legal meanings determined by one or more court decisions. These meanings may be different from common usage. For instance, the word "comprises", when used in the claims of a United States patent, means "consists at least of". By contrast, the word "consists" means "consists only of", which will lead to a very different scope of protection.