Congress delegates rule-making authority to the Department of the Treasury in Internal Revenue Code § 7805(a) and other Code sections. Thus, the Treasury department promulgates regulations (also called "rules") pursuant to statutory authority. (The Internal Revenue Service (I.R.S.) is part of the Treasury Department.)
Regulations are the highest administrative authority issued by the Treasury Department (higher than Revenue Rulings and other guidance documents).
Before the Treasury Department can issue regulations in final form, it must first publish proposed regulations and allow the public time to comment on them. Unlike proposed regulations, temporary regulations go into effect immediately upon publication in the Federal Register, but they must expire within three years of enactment (with the exception of temporary regulations issued prior to 1989). In contrast, final regulations are permanent and are promulgated after the public has had an opportunity to comment upon them.
Temporary and final regulations have the force of law, while proposed regulations generally do not (except that proposed regulations can be cited as substantial authority for avoiding the understatement of income tax liability under I.R.C. § 6662(b)(2)).
["Seal on United States Department of the Treasury on the Building" photo by MohitSingh, Creative Commons license: CC BY-SA 3.0]
Find temporary and final treasury regulations arranged by topic in Title 26 of the Code of Federal Regulations. To find proposed regulations, use the Federal Register (which publishes all three types of regulations and is arranged chronologically by publication date).
For a more detailed discussion of how to research federal regulations, see USF's Federal Regulations Research Guide.
You can find Treasury Regulations in these resources:
Title 26 of the Code of Federal Regulations (C.F.R.) is the Internal Revenue title. Instead of citing them like other regulations (for example, 15 C.F.R. § 10), however, the BlueBook cites them as Treasury Regulations. The BlueBook citation also makes apparent the type of regulation (final, proposed, or temporary), as the following examples illustrate: Treas. Reg. § 1.61-2(c), Prop. Treas. Reg. § 1.704-1, and Temp. Treas. Reg. § 1.71-1T.