Rojas’s case is governed by Illinois law, which “mandates that when ‘different instruments are executed together as part of one transaction or agreement, they are to be read together and construed as constituting but a single instrument.’ ” IFC Credit Corp. v. Burton Indus., Inc., 536 F.3d 610, 614 (7th Cir. 2008) (quoting McDonald’s Corp. v. Butler Co., 511 N.E.2d 912, 917 (Ill. App. Ct. 1987)); see also Gallagher v. Lenart, 874 N.E.2d 43, 58 (Ill. 2007) (“[I]nstruments executed at the same time, by the same parties, for the same purpose, and in the course of the same transaction are regarded as one contract and will be construed together.”). And if a contract incorporates another document by reference, the “additional provisions become as much a part of the contract as if they were expressly written in it.” Westlake Fin. Grp., Inc. v. CDH-Delnor Health System, 25 N.E.3d 1166, 1172 (Ill. App. Ct. 2015) (quoting Wilson v. Wilson, 577 N.E.2d 1323, 1329 (Ill. App. Ct. 1991)). These principles of Illinois law defeat Rojas’s argument. He concedes that he signed the buyer’s order and installment contract as part of the same transaction. Moreover, the buyer’s order explicitly incorporates any retail installment sale contract and specifies that the agreement will not remain binding if a third party finance source does not agree to purchase the installment contract. Thus, under Illinois law, we must construe the documents together. Rojas replies that that the agreements conflict, and therefore should not be read together, because one refers to the financing condition and the other does not. But the agreements do not contradict each other; one simply adds a condition that the other does not forbid. See Tepfer v. Deerfield Sav. & Loan Ass’n, 454 N.E.2d 676, 679 (Ill. App. Ct. 1983) (construing documents together when “there are any provisions in one instrument limiting, explaining, or otherwise affecting the provisions of another”); IFC Credit Corp., 536 F.3d at 615 (reading contracts together even though unconditional obligation in one was subject to condition precedent in another); see also Janikowski, 210 F.3d at 768 (reading the purchase order, containing the financing condition, with the installment contract as one agreement). When the two consistent documents are read together, the disclosures in the installment contract comply with TILA.