of Turing degrees, either A includes a cone or the complement of A contains a cone. It is similar to Wadge's lemma for Wadge degrees, and is important for the following result.

We say that a set of Turing degrees has measure 1 under the Martin measure exactly when contains some cone. Since it is possible, for any , to construct a game in which player I has a winning strategy exactly when contains a cone and in which player II has a winning strategy exactly when the complement of contains a