It's public domain, see also: http://patents.stackexchange.com/questions/5766/is-an-expired-patent-in-the-public-domain http://patents.stackexchange.com/questi

It's public domain, see also: http://patents.stackexchange.com/questions/5766/is-an-expired-patent-in-the-public-domain http://patents.stackexchange.com/questions/5766/is-an-expire... The idea of a patent is that for a given number of years your invention is protected, even if public, you cannot re-patent it "as is" (as it would not be anymore new or non-obvious) but you can sometimes patent a derivative (like an improvement of the same base idea). The generic issue may be that often behind something there can be a number of patents, often interlinked, so it is far from easy to determine whether you can "use" that expired patent.