810 ILCS 5/2A-204: Formation in general.

(1) A lease contract may be made in any manner sufficient to show agreement, including conduct by both parties which recognizes the existence of a lease contract.

(2) An agreement sufficient to constitute a lease contract may be found although the moment of its making is undetermined.

(3) Although one or more terms are left open, a lease contract does not fail for indefiniteness if the parties have intended to make a lease contract and there is a reasonably certain basis for giving an appropriate remedy.