In Amberboy v. Societe de Banque Privee, the supreme court, answering a certified question from the United States Court of Appeals for the Fifth Circuit, held that a note with a variable rate of interest determinable only by reference to a bank's published prime rate is a negotiable instrument under the Texas Business & Commerce Code (the Code). See 831 S.W.2d 793, 797 (Tex. 1992) (interpreting when a writing is a negotiable instrument under former section 3.106 of the Business & Commerce Code). Although the comment to former section 3.106 of the Code stated that an instrument was negotiable only if the sum certain to be paid was capable of computation "from the instrument itself without reference to any outside source," the supreme court overruled this "four corners" rule of negotiability. Id. at 794 (quoting comment 1 to former section 3.106 of the Business & Commerce Code). The court held that the Amberboy ruling promoted the Code's fundamental purpose to "simplify, clarify and modernize the law governing commercial transactions." Id. (quoting former section 1.102(b)(1) of the Business & Commerce Code). In so doing the court noted that when the Code was adopted, variable interest rates were unknown; since the 1980s, they have come to dominate modern commercial practices, and a construction of the Code that recognizes that commercial certainty can be accomplished by reference to some authority outside the writing itself "serves the purpose of the law of negotiable instruments, which is to make the instrument the functional equivalent of money." Amberboy, 831 S.W.2d at 796. The Amberboy court limited its holding to notes with a variable interest rate that "is readily ascertainable by reference to a bank's published prime rate." Id. at 797.