The most basic lawsuits worry just two people, the plaintiff and the defendant. But what if you’re worried that someone else’s lawsuit is going to affect your rights?

The Federal Rules of Civil Procedure allow intervention in a lawsuit in several ways. One way is through Rule 24(b), permissive intervention. Under Rule 24(b), anyone can intervene if they have “a claim or defense that shares with the main action a common question of law or fact.” The Sixth Circuit recently gave one example of what’s not “a common question of law”: when a group wants to intervene to protect the interpretation of one law, but the suit the group wants to join is about a different law.

In 2010 Bay Mills Indian Community opened a casino in Vanderbilt, Michigan, and was promptly sued by the state government. Michigan claimed that the casino violated the Indian Gaming Regulatory Act and the gaming compact between Michigan and Bay Mills. Together these restrict casinos to “Indian land.” So Bay Mills sued Michigan right back, citing the Michigan Indian Land Claims Settlement Act. Bay Mills argued the casino was legal because the settlement act describes the land where it sat as land “held as Indian lands are held.”

Seven years later, the Saginaw Chippewa Indian Tribe of Michigan moved to intervene under Rule 24(b). The Tribe argued there was a common question of law because Bay Mills’s suit could affect how courts define “Indian land” under federal law—federal law that also applied to the Tribe. The Western District of Michigan held this wasn’t enough for a common question of law, and the Sixth Circuit affirmed.

The Sixth Circuit reasoned that the case didn’t involve the Indian gaming act at all—it involved the settlement act, and its reference to lands “held as Indian lands are held.” The Tribe wasn’t a party to the settlement act so it couldn’t be affected by any interpretation of that act. “[A]ny effect on the interpretation of [the Indian gaming act] would be a mere side effect. Allowing this to fulfill the ’common question’ requirement would mean that any Indian tribe would be able to intervene whenever IGRA is implicated in any way—or, to analogize, it would allow any employer to intervene in any Title VII suit.”

The Sixth Circuit’s decision is here.