The US patent in question, number 5,933,841, is entitled 'Structured Document Browser' and was granted in 1999 to SBC Communications.
According to the US Patent Office's web site, this invention includes "a constant user interface for displaying and viewing sections of a document that is organised according to a pre-defined structure."
The browser, according to the patent filing, "displays documents that have been marked with embedded codes that specify the structure of the document. The tags are mapped to correspond to a set of icons. When the icon is selected while browsing a document, the browser will display the section of the structure corresponding to the icon selected, while preserving the constant user interface."
This, SBC claims, covers internal links that stay visible on a web site during navigation – a form of site navigation used in millions of sites. The patent has not been tested in court – yet.
However, SBC reportedly sent a letter to the business behind the US education product site museumtour.com, claiming that the site's navigation system infringes "several issued claims" in the patent, and asking the company to pay licensing fees.
According to the letter, the base rate for a licence is $5,270 per year for a company with a $100,000 annual revenue, rising to $16 million for a $10 billion company, ZDNet UK reports.
The text of the patent is available here