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I'm asking in particular about this patent.

The filing year was 1991, so it is currently 23 years old. However, the fee status says "paid." Google Patents also does not mention anything about that patent being expired.

Is that patent expired, and if so, why does it not say so?

4

The term of a patent is 20 years from the filing date. The legal status of the patent will be "expired" in the case of a patent filed in 1991,. This is because the term of the patent has been more than 20 years.

Further, once a patent is granted, it is must for a patentee to pay the prescribed renewal fees to keep the patent in force until the completion of the patent term. The procedure and rules for payment of the renewal fees varies from jurisdiction to jurisdiction. It has to be noted that non payment of the renewal fees will lead to revocation of the patent.

http://www.invntree.com/blogs/can-lapsed-patent-be-restored-india

Further, the fee payment status is being shown as paid because renewal fees of the patent has been paid till the patent was in force.

2

This patent has expired.

There's no record because the USPTO doesn't change expired patents' statuses, but it definitely is. Since it was before 1995, it would either have expired after 17 or 20 years, and you're fine in either case.

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    Think of a published patent as images and words on several sheets of paper. Once printed and distributed the words on the page do not change. A PDF of a patent document reflects that unchangable paper. The patnt office can list expired patents in the Gazzette but the published document is fixed. – George White Nov 6 '14 at 3:46
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The patent has already expired since it is more than 20 years. And once a patent term has ended it is not renewable. But just because a patent has expired doesn't mean it is already a public domain. Search if the owner has filed a continuation or improvement patent similar to the expired one. This is mostly done by inventors to extend their exclusive rights on the patent.

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    A continuation would use the original priority date, and so continuation and continuation-in-part patents would also be expired. – vallismortis Sep 5 '15 at 16:43

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