The Internal Revenue Code in Title 26 of the United States Code establishes the tax code and its regulations, according to IRS.gov. For example, Title 26, Subtitle A, Chapter 1, Subchapter A section 3 of the code authorizes tax tables for individual taxpayers who itemize their deductions.
The tax code is interpreted by Treasury code, according to IRS.gov. Title 26, Subtitle A, Chapter 1, Subchapter A, Part I, section 2 defines the terms "surviving spouses" and "heads of household" for filing purposes. The section states that spouses who have remarried do not qualify as surviving spouses, with some exceptions. This part of the tax code also deals with things such as nonresident aliens and military personnel who are missing in action.