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184.3. Control can be Contested through the course of play.

184.3.a. Contested is a temporary status applied to the battlefield when a Unit controlled by a Player who does not currently Control that Battlefield Moves or otherwise becomes present there.
184.3.a.1. Units moving to or being played to a battlefield apply Contested status if that battlefield is not already contested and that Unit ’s controller does not already control that battlefield.
184.3.b. A Battlefield remains Contested until Control is established or re-established.
184.3.c. While a Battlefield remains Contested, Control of a Battlefield cannot change.
184.3.d. The state of a Battlefield being Contested is used to determine when Combat should occur, when a Showdown without a Combat should occur, and when Control will change.
184.3.e. At this time Game Effects cannot reference this status.