how to know if a patent expired or not?

发信人: xiaohuoguo (guoguo), 信区: StartUp
标  题: Re: how to know if a patent expired or not?
发信站: The unknown SPACE (Thu Mar 21 10:07:18 2002) WWW-POST

Usually, a patent expires after 20 years from its filing date.  Because the
maintainence fee grows every period, such as first three years $400, second
three years &800, and so on.  It is a big burden for some companies if it
maintains many patents at the same time, so some companies just abandon their
patents without paying maintainence fees.  I think but not sure that the
status of a patent is not published on US patent and Trademark Office website,
but you can make an order asking USPTO to check the status of patent for you
with some fee.