There is a principle of patent invalidation: the patent is invalid from beginning to end. That is, the patent right is equivalent to never appearing. Originally, the date of calculation of patent rights is the filing date (if there is priority, the priority date), but once it is invalid, it is equivalent to the patent being invalid from the beginning. But that doesn’t mean that an expired patent is useless. It can be consulted by examiners and related technical personnel as a patent document. In addition, such a technology is disclosed in writing. Patent documents are generally recorded in detail. So it is convenient for people to know such a technology. Thank you. I wonder if you can explain it clearly?