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"Means plus function" refers to a type of patent claim that does not specify a particular structure for an invention, but instead describes a "means" for achieving some function. The form was made available by the U.S. Patent Act of 1952: before 1952, patents could only be granted to specific machines.

A means plus function claim looks like this:

A communication apparatus for transmitting and receiving analog voice signals, digital data signals, and data carrier signals that precede and follow the digital data signals to and from a communication medium, in the same session, said apparatus comprising: telephony means for transmitting and receiving said voice signals to and from said medium; modem means for transmitting and receiving said data signals to and from said medium; switch means interposed between said communication medium, said telephony means, and said modem means...
Cygnet Technologies "Digital and voice telecommunication apparatus," U.S. Patent No. 4,524,244 (1985).

According to the abstract, this patent is referring to a system that allows the transmission of "analog voice and digital data signals to and from a telephone line (in) the same session." But note that the word "telephone" appears nowhere in the actual claim, which is the legal "meat" of the patent, determining what infringes and what doesn't. Inasmuch as we have the words "telephony" and "modem," they're being used as adjectives.

This is done is to prevent others from skirting the patent's scope by changing the physical components of the invention but retaining the same idea. If the claim replaced "telephony means" with the noun "telephones," the patent could logically be construed to extend only to actual, physical telephones. Someone could replace the telephones with computers, walkie-talkies, tin cans, or whatever they could engineer into the system, and they would not be violating the patent. Or, if the telephone suddenly became obsolete in favor of a new, ultra-advanced voice communications device, the fancy device could be linked into a system otherwise identical to the patent's claim, and the patent would effectively die an early death.

Note that while the claim can be broad and rather non-descriptive, the patentee still has to have a physical invention to show for their efforts, and the other parts of the patent have to illustrate how that invention is built. In the above case, the patent description outlines a box hooked up to an IBM PC through its RS-232 port. But assuming the patent were still in force, you couldn't sell a similar device inside a USB keychain that works with a PowerBook G4. It would probably be better than what Cygnet developed, but it would be a patent infringement, since it still follows the means and function of the claim.

However, the terms of the claim still have to be construed strictly. This means that you could make an otherwise identical box that works with digital voice signals, instead of analog voice signals, and that would be outside the scope of the patent.

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