What is the rule of res judicata?

The rule from Henderson v. Henderson is an extension of the policies that underlie res judicata, and it adds the idea that parties and privies are obliged to bring forward their whole case with respect to a matter in dispute to avoid duplicative litigation. The often quoted statement of this extension of res judicata comes from the judgment of Wigram, V.C. in Henderson v. Henderson (1843), 3 Hare 100 at p. 115 where he stated: The plea of res judicata applies, except in a special case, not only to points upon which the court was actually required by the parties to form an opinion and pronounce a judgment, but to every point which properly belonged to the subject of the litigation, and which the parties exercising reasonable diligence, might have brought forward at the time.” [emphasis added]