28 U.S.C. § 3305 - U.S. Code - Unannotated Title 28. Judiciary and Judicial Procedure § 3305. When transfer is made or obligation is incurred

(A) with respect to an asset that is real property (other than a fixture, but including the interest of a seller or purchaser under a contract for the sale of the asset), when the transfer is so far perfected that a good-faith purchaser of the asset from the debtor against whom applicable law permits the transfer to be perfected cannot acquire an interest in the asset that is superior to the interest of the transferee; and

(B) with respect to an asset that is not real property or that is a fixture, when the transfer is so far perfected that a creditor on a simple contract cannot acquire, otherwise than under this subchapter, a judicial lien that is superior to the interest of the transferee.

(A) if oral, when it becomes effective between the parties; or

(B) if evidenced by a writing executed by the obligor, when such writing is delivered to or for the benefit of the obligee.