Does the US recognize common-law marriage?
Only about 8 states currently recognize common-law marriage formed within their borders: Colorado, Iowa, Kansas, Montana, Oklahoma, Rhode Island, South Carolina, Texas, and Utah, plus Washington DC. Requirements vary but generally require: cohabitation, intent to be married, and holding yourselves out as a married couple.
Palimony is a term for financial support awarded after a long-term unmarried relationship, based on an express or implied contract between the partners. It originated in Marvin v. Marvin (1976) in California. Not all states recognize palimony claims, and they are much harder to prove than spousal support.
What should unmarried partners do to protect themselves?
The single best protection is a cohabitation agreement — a written contract that specifies property rights, financial contributions, and what happens if the relationship ends. It is enforceable as a contract in most states. Without one, unmarried partners generally have no automatic financial rights.