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A patent gives the holder the right to exclude others from making, using, or selling an invention defined by its claims, once granted by a government office tasked with this chore. It's a common misconception that a patent holder has the right to practice his invention: this isn't always true, for example in the case of an improvement on an earlier invention. Patents are territorial: there is no such thing as a "world patent", although the World Intellectual Property Organization does provide a streamlined filing system for international applications.
Albert Einstein, famously, was a patent clerk, which just goes to show ya.
Patent rights vary in duration, and in what efforts the owners must go to in order to retain them (usually: paying renewal fees; sometimes: working the invention). In most countries, including the United States, patents last for 20 years from the date the application is filed; under previous US law, they lasted for 17 years from the date of issuance. There are often provisions for extending patent term, if the holder had to obtain regulatory approval before marketing a product (most often, a pharmaceutical). In most countries, patents are only granted for new, useful, and non-obvious inventions; however, the legal provisions governing each of those terms can differ. Obviousness is determined from the standpoint of a person having ordinary skill in the art.
Patent drafting, prosecution (i.e. securing grant) and litigation is skilled work, as is that performed by patent examiners. Patent lawyers often have scientific or engineering qualifications; this is a requirement in some countries.
 Patents and conspiracy theories
Patents figure in many conspiracy theories, especially those regarding automotive woo. A typical argument goes that we cannot buy cars with, e.g., 200-mpg carburetors because oil companies have bought the patents. That argument suffers from many flaws:
- If true, it would be insanely easy to prove: Just give the patent numbers.
- Such rumors have been around long enough that the patents would have expired by now.
- Since few if any inventors deal with the bother and expense of obtaining patent protection around the world, people at least somewhere in the world should be free to make and sell such cars.
- In most countries, patent applications are published 18 months after the initial filing, after which it's a bit too late for oil companies to suppress the information.