Scope of a Patent
Just as the scope of real property is determined by looking to metes and bounds, the scope of a patent is determined by the claims. And just as the language used in metes and bounds is very specific, so is the language used in drafting claims. The scope of a patent is determined by looking to the interpretation of those claims and how they relate to other parts of the specification.
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Interpretation
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The interpretation of individual words in a patent claim often raises tricky issues. There are competing ways of interpreting individual words. For example, section 2111.01 of the Manual for Patent Examining Procedures (MPEP) states "the words of a claim must be given their 'plain meaning' unless such meaning is inconsistent with the specification." This section expands on this, saying that the "plain meaning refers to the ordinary and customary meaning given to the term by those of ordinary skill in the art". While you would think this straightforward, the MPEP provides that this presumption can be rebutted because an inventor may be his own lexicographer. This has led many patent drafters to include a glossary in the patent application.
Doctrine of Equivalents
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The doctrine of equivalents also pertains to interpreting the scope of a claim. Under this doctrine, if a patent invention claims a particular element, anything equivalent to that element falls within the scope of the invention. For example, if an invention includes an incandescent light bulb, variations of the same invention that include a halogen or fluorescent light bulb could be arguably equivalent.
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The Festo Case
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The doctrine of equivalents came under attack in the 2002 Supreme Court case Festo Corp. v. Shoketso Kinzoku Kogyokabushiki Co. In this case, the court ruled that the doctrine of equivalents does not apply if a claim was amended so as to make the invention patentable. The doctrine of equivalents continues to apply if a claim is amended to clarify it.
Amending Claims
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The Festo decision sent shockwaves through the normally staid world of patent law. It made many patent attorneys and agents think twice before amending claims because they did not want to narrow the scope of an invention. It also encouraged patent practitioners to draft both broad and narrow claims to best protect the invention. Returning to the light bulb example, one might broadly claim a "lighting means." Additional claims for specific types of light bulbs would also be added. In this way, the scope of the invention would not suffer too much if one claim was not allowed.
Warnings
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Questions regarding the scope of a patent can be enormously complicated. Claim drafting and claim interpretation are often best left to patent agents and attorneys because skill and experience are needed so that the scope of the patent is as broad as possible while still being defensible.
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References
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