In order for a patent claim to be valid, it must propose a concept, idea, or item that is "useful," "novel," and "nonobvious." Below are definitions of these terms in the context of the patent process.

"Useful" - The term "useful" means that the subject matter has a useful purpose. It also requires that the item is operable, since a machine that can not perform its intended purpose cannot be considered useful in the ordinary sense of the word.

"Novelty" - "Novelty" is strictly defined by patent law. An invention cannot be patented if:

• The invention was known or used by others in the United States before the patent applicant invented it.

• The invention was patented or described in any printed publication, before the patent applicant invented it.

• The invention was patented or described in a printed publication in any country more than one year prior to the inventor's U.S. patent application.

• The invention was in public use or on sale in the United States more than one year prior to the inventor's U.S. patent application.

These rules do not prevent a person from patenting an improvement to another invention, however. For example, tire makers have long known the formulas for making tire rubber. But what if an inventor found a way to make tire rubber twice as long-lasting by slightly changing the chemical composition? This could well be a patentable improvement as long as the difference was not obvious.

"Nonobviousness" - Even if a new invention differs in one or more ways from another patented invention, a patent may still be refused if the differences would be obvious. Nonobviousness is defined as a sufficient difference from what has been used or described before that a person having ordinary skill in the area of technology related to the invention would not find it obvious to make the change. For example, sodium chloride (table salt) and potassium chloride (a chemically similar salt) can often be used interchangeably. A chemist working to improve road salt would consider it obvious to substitute potassium chloride for sodium chloride, so a formula that simply made this substitution in an already patented road salt formula would not be patentable.