Qui tam

Short for 'qui tam pro domino rege quam pro se ipso in hac parte sequitur' ('who brings the action for the King as well as for himself.'). Oddly, it is found predominantly in the United States, which chose to do without a king some time ago. If for 'king' one reads 'government', it makes more sense. The qui tam relator sues to enforce the government's rights, and the action would be captioned 'John Doe, ex rel United States of America v. Defendant', which makes clear the relationship between the relator and the true party in interest - the US government. Legislation creates a financial incentive to encourage private litigants to bring actions to redress harm to the government.

In the IP world what is often held out as the paradigm example is found in the US Patent Act, Section 292, which provides:

(a) . . .
Whoever marks upon, or affixes to . . . any unpatented article, the word “patent” or any word or number importing that the same is patented, for the purpose of deceiving the public
. . .
Shall be fined not more than $500 for every such offense.
(b) Any person may sue for the penalty, in which event one-half shall go to the person suing and the other to the use of the United States.

(emphasis added). In Stauffer v. Brooks Brothers, Inc. v. United States (Fed. Cir. 2010) an action arose from a mark on bow ties which featured a once-patented adjustment mechanism (the patents having expired in the 1950s: here is a link to Barbara Cookson's photo of an example, from my own collection of neckwear, bought in New York in 1991 so probably too late for  me to claim now). However, this is not a true qui tam action (note that Mr Stauffer was not the relator, but the plaintiff) because the Government gets a share of the plaintiff's recovery. It is similar, but the express language gives standing to 'any person.'