Siri is designed to understand and respond to questions and requests, such as queries about facts or nearby businesses, expressed in the user's own words. It was integrated into Apple's iPhone 4S, introduced last October, but a version of it previously had been available as a third-party app for other iPhones. It is offered only on the 4S, the latest model of the iPhone, and Apple has used it as a major selling point for the handset.

Potter's patent, entitled "Method and apparatus for controlling a digital computer using oral input," lays out a system for interpreting commands expressed in normal spoken language. The idea behind it was to cut out the sometimes lengthy training that's needed for traditional voice-control systems that require specific commands. This could make it easier for new and casual users to take advantage of voice control, according to the introduction to the patent.

After spoken words are received by a microphone and interpreted by voice recognition, those words are used to search the contents of a tabular data structure organized in rows and columns, the patent says.

Because the patent application was filed in 1995, Potter probably couldn't use it to collect royalties beyond 2015, according to Colleen Chien, an assistant professor of law at Santa Clara University in Santa Clara, California. However, the company could still win damages for past infringement, she added. The age of the patent could also work in its favor, because it predates by many years the introduction of Siri.

The defendants in the case might attack Potter's suit on the basis of the America Invents Act, an overhaul of the U.S. patent system signed into law . Among other things, the new law is designed to make it harder for plaintiffs to go after many different parties in the one suit for patent infringement, said Eric Goldman, an associate law professor at Santa Clara. However, suing several parties in the same action can end up benefitting either side, he said.