Federal tax laws come from four sources:

• Tax treaties agreed to and signed between the United States and another country.
• Legislation enacted by the U.S. Congress (the U.S. Senate and the U.S. House of Representatives), and signed into law by the  President. Tax laws become part of the U.S. Code, of which Title 26 is the Internal Revenue Code (the "IRC" or "the Code").

• Administrative laws

The U.S. Department of Treasury is the executive department responsible for implementing tax laws.

The Internal Revenue Service (IRS), a bureau within the Treasury Department, issues documents for interpreting the IRC and enforcing tax laws--regulations to letters commenting on specific transactions. Many of these documents are called regulations.

• Judicial decisions are reached by opinions issued by a court.  Courts include the Supreme Court of the United States, the U.S. Tax Court, the Court of Federal Claims, Federal Appellate Courts, and federal district courts.

The major tax codifications are the IRC of 1939, 1954, 1986 and 2018.

Annual editions have been published.

• Tax has many "terms of art" and some ordinary words, such as "person" have special meanings.
•   Look for defintions that apply to a section or sub-section.  §7701 of the IRC has more than 50 definitions of commonly used terms.
• When an agency other than the Treasury Department has primary responsibility for an area of law, you will need to check laws of that agency that relate to tax.