U. S. Constitution
Code of Federal Regulations
Federal Register ( http://www.access.gpo.gov:80/su_docs/aces/aces140.html )
Legislative Branch U.S. Code
The US Code from Cornell is the easiest online version to use. If this site is too busy, try the House of Representatives site.
Supreme Court Opinions
Includes both current opinions (since 1990) and those deemed historic (580 of most important older cases).
Federal Circuit Court Opinions
Includes a clickable map of the federal circuits
and U.S. Supreme Court to provide easy access to court decisions.
United States District Court: District of Massachusetts Site began in December 2015.Currently has cases since November 2016.