The Federal Circuit today ruled that the Eastern District of Texas has the wrong idea about whether Rule 20(a) lets plaintiffs sue a bunch of defendants in one patent infringement case simply because they all infringed the same patents.

You have to show something more, it held — such as that the defendants somehow acted in concert to infringe the same patent, made the same infringing product, or used the same infringing method to make their own infringing products. In re EMC Corp., Misc. No. 100 (Fed. Cir. May 4, 2012) (granting mandamus and ordering district court on remand to use different legal test in ruling on motions to sever and transfer).

Blawgletter notes that the ruling applies to mandamus petitions that pre-date the Leahy-Smith America Invents Act, which in 2011 changed the joinder rules in patent cases.