Treasury regulations are the IRS's official and authoritative interpretation of the IRC. Regulations are first published as "proposed" regulations, which allows for a public comment period before the regulations become final. Final regulations are issued as Treasury Decisions (TD's) and are numbered in order of their issuance.
The IRS may also issue temporary regulations when immediate regulatory action is needed. Temporary regulations are effective immediately upon publication and remain in effect until final regulations are issued. Temporary regulations are issued and numbered as Treasury Decisions (TD's).
Proposed regulations and Treasury Decisions can be found in:
Treasury regulations are codified in Title 26 of the Code of Federal Regulations, but they are usually cited as "Treas. Reg." rather than by the CFR title. Current regulations can be found in:
For regulations in force in prior years, see West's Federal Tax Regulations (1954 to 2002).
Most IRS documents are published in the weekly Internal Revenue Bulletin ("IRB") (also online). The IRB is cumulated twice a year and published as the Cumulative Bulletin (also on HeinOnline). Some documents, like private letter rulings, are not officially published but are available in other sources. IRS pronouncements published in the IRB are binding authority and must be followed by the IRS. The current status of IRS pronouncements should be verified with an online citator, either through Lexis, Westlaw, or one of the major looseleafs.