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When I researched the assignment history of a patent I found one assignment (from assignor A to assignee B) and a later assignment (from assignor B and C to assignee D).

Where does C come from?

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Providing the actual patent number would help others to review that particular assignment information. – Ron J. Feb 13 '13 at 11:28
Try 6535909. See Assignments 5 & 6 or 6 & 7? Where do the additional entities come from? – POe Feb 13 '13 at 12:07

The simple answer is that the USPTO Assignment Record Database is a complete mess and cannot really be relied upon to point you to the correct owner. There are many "off record" transactions. In your case, the assignment from "C" may also be something like a quitclaim deed.

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