35 U.S.C. Sec 103(a)
United States Code Title 35 - Patents, Section 103 Conditions for Patentability; non-obvious subject matter.
(a) A patent may not be obtained… if the difference between the subject matter sought to be patented and the prior art are such that the subject matter as a whole would have been obvious at the time the invention was made to a person having ordinary skill in the art…