The case of Meister v. Moore (96 U.S. 76 (1877)), helped define the concept of the common-law
marriage. Basically, a marriage is a contract, and under the common
law, contracts can be entered into in a variety of ways, including informally.
A common-law marriage is a valid
contract entered into in an informal manner, much like verbal contract. It is
still valid an enforceable.
The Court noted that a Statute
can explicitly ban common-law marriage
and require that all marriages be solemnized (kind of like how the Statute of Frauds
requires some contracts to be in writing). However, if the State doesn't
specifically forbid common-law marriage, then it is permissible.
The basic rule is that if
the State does not say 'no', then common-law marriage is available.
Today, 3/4th of the
States do not permit common-law marriage, and have explicit Statutes against the practice.