The Internal Revenue Service (IRS) is a bureau of the Treasury Department and ultimately is responsible for the collection of taxes and enforcement of the Code. Pursuant to this authority, the IRS provides guidance to taxpayers, tax professionals and IRS personnel through a variety of documents. They are: 

1. Revenue Rulings (Rev. Rul)
2. Revenue Procedure (Rev. Proc.)
3. Notices 
4. Announcements (Ann.)
5. Private Letter Ruling (PLR) 
6. Determination Letters
7. Technical Advice Memoranda (TAM)

Descriptions of these documents are provided in the IRS Primer.

The above documents have legal authority and they are described more fully in the IRS Primer. However, there are other guidance documents that do not have substantial legal authority but can be helpful for understanding the IRS's position on relevant issues. These are: 

1. Chief Counsel Memoranda
2. IRS Forms and Instructions
3. Internal Revenue Manual - the manual that IRS employees use in audits 

To promote uniform application of tax law, the IRS makes pronouncements in the Internal Revenue Bulletin, which is published weekly. 

The Bluebook has rules for citing to Department of the Treasury materials. Please see Table TI.