The Internal Revenue Service, in the executive branch of government, promulgates (announces publicly) rules by publishing them in the Federal Register. This being done, a rule becomes a regulation with the force law, and it is codified it the Code of Federal Regulations. Regulations are also accessible in some research databases. Here are links to a few access points.
A regulation is promulgated by the Internal Revenue Service and Treasury Department in the Federal Register to get the force of law. They provide guidance for new legislation or address issues that arise with respect to existing Internal Revenue Code sections. Regulations enforce and give directions on complying with a statute.
Both proposed and final regulations are published in the Federal Register, and final regulations are codified in the Code of Federal Regulations. In addition, they are printed under the heading "Treasury Decisions" (T.D.) in the Internal Revenue Bulletin .