Inventors want the maximum possible protection over their inventions. They must therefore understand how patents conferred by the U.S. Patent and Trademark Office will provide that protection. The heart of a patent is its so-called "claims."
Patent claims establish the boundaries or scope of an invention. They are the standard by which patent rights are measured. In other words, when a patent owner sues for infringement, the basis of that lawsuit is that someone has made, used, or sold an invention that has all of the elements in one of the claims, or that closely fits the description in the claims. In this manner, claims function like the boundaries in a deed for real estate. The claims are subject to rigorous examination during the patent application process (known as patent "prosecution"). How do these claims work?
The scope of patent coverage—that is, how narrow or broad the claims—is determined by the novel features that distinguish an invention over prior art and provide new results that are different or unexpected enough to be considered nonobvious.
The fewer the novel features needed to distinguish an invention, the broader the scope of its coverage. Stated differently, if many new features are needed to distinguish the invention from prior art, the coverage is narrow and it's usually easier for a competitor to provide the same results without infringing.
Note that patent claims can be highly technical. You can view sample claims for common inventions, though the description of the claims can be far more intricate when the invention is, for example, a complex machine or pharmaceutical product. (Whether you decide to write your claims yourself or hire a patent attorney to assist you may depend on the complexity of your invention and your familiarity with technical writing.)
Patent are usually made up of independent and dependent claims. One claim is stated as broadly as possible (the "independent claim") and then followed successively with narrower claims designed to specifically recite possible variations ("dependent claims").
The independent claim stands by itself, while a dependent claim always refers back and incorporates the language of another independent or dependent claim (see 35 U.S.C. § 112(3) and (4)). Below is an example of an independent and dependent claim for a golf club and bag security system (U.S. Pat. No. 5,973,596). In this example, the independent claim defines the elements of the golf bag security system and the dependent claim recites one aspect of it more specifically by stating that the alarm can be turned on and off by a separate device.
In an infringement case, a court examines the claims of the patented invention and then compares them to the defendant's device or process. The court determines whether the claims read on (or cover) the defendant's device or process. To be found to infringe a patent, the defendant's device must physically have or perform all of the elements contained in one of the claims.
For example, if a patent claim recites two elements, (i) a hidden pocket in a scarf, and (ii) a snap that makes the pocket detachable, a device that contains only a hidden pocket in a scarf won't infringe.
A dependent claim cannot be infringed unless the allegedly infringing invention also infringes the related independent claim. In other words, if an independent claim is not infringed, then the dependent claims cannot be infringed.
For a more general review of patent law, see Nolo's Patents for Beginners by David Pressman and Richard Stim.