Treasury Regulations are the official interpretation of the Internal Revenue Code by the Treasury Department. Regulations are published in the Federal Register when they are issued, and then printed in the Internal Revenue Bulletin (IRB) the week that they are issued. The Internal Revenue Bulletin used to be consolidated into the Cumulative Bulletin (CB) on a semiannual basis, but the CB ceased publication in 2008. The regulations are ultimately codified into Title 26 of the Code of Federal Regulations (CFR).
The numbering of the Treasury Regulations closely follows the numbering of the Internal Revenue Code, except with the addition of a prefix before the Code section number and a suffix identifying the regulation. The prefix number (e.g., 1.xx) identifies the citation as a regulation, and also signals the type of tax involved (for example, 1 is income tax, 20 is estate tax, 25 is gift tax, 301 is for procedural matters, and so on).
Aside from the regulations, agency documents are also an important administrative source of federal tax law. These include revenue rulings, which explain how the law applies to a specific factual scenario, revenue procedures, which explain the internal practices and procedures of the IRS, and letter rulings, which describe how the IRS will treat a future transaction, and are applicable only to the taxpayer who asks for the ruling. There are several other types of agency documents, but these are some of the key ones.