Common law marriage—also known as marriage without formalities or informal marriage—is marriage without a wedding or marriage ceremony and without a marriage license. Some states recognize common law marriage under certain circumstances—such as when the parties agree to be married, live together (cohabit) after agreeing to be married, and represent to others that they are married (holding themselves out as a married couple).

Other states previously recognized common law marriages, but no longer recognize such marriages after a certain date on which the law was changed—usually by statute. In other states, common law marriage has not been recognized by the legislature in a statute, but the courts have recognized common law marriage in court opinions or case law.

In Montana, common law marriage is recognized under certain circumstances. To establish a common law marriage in Montana, the couple must meet several requirements: both parties must have the capacity to consent to the marriage, both must agree to be married, they must cohabit, and they must have a reputation in the community as being married. There is no specific time frame for cohabitation required. Once a common law marriage is established, it is legally recognized in the same way as a ceremonial marriage. This means that the couple will have the same legal rights and obligations as a couple who had a formal wedding. If the couple decides to end their relationship, they must go through the same legal divorce process as any other married couple. It's important to note that not all states recognize common law marriages, and the recognition of a Montana common law marriage may vary if the couple moves to another state.