What is the most important law in marriage?

Marriage laws are determined at the state level, although federal courts have intervened throughout history to ensure equal protection under the law. For example, the U.S. Supreme Court ruled in 2015 that states cannot restrict the institution of marriage to just heterosexual couples under the Equal Protection clause of the 14th Amendment to the Constitution.

State marriage laws determine the age at which an individual can get married with and without parental consent; whether blood tests are required; how marital property is divided in the event of a divorce; and other considerations. State laws also have criteria for what constitutes an invalid marriage and thus eligible for annulment.