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Marriage is a relationship in which 2 people enter a legally-binding agreement in which they share many aspects of their personal lives with one another, typically out of romantic love. In many states, marriage is defined as being between one man and one woman, but a small (but growing) minority allows two people of the same sex to get married.
Gender-based restrictions aside, the requirements for getting married are pretty similar from state to state. Generally, the 2 parties must be over the state's age of majority (usually 16-18 years old), neither of them must be married to another person, and they must not be more closely related than first or second cousins (the cutoff varies between states).
When two people are married, they assume many legal rights and responsibilities with respect to one another. They are bound by a duty to care for each other, for example. They also usually get power of attorney over each other, allowing one to make legally-binding decisions for the other in case they're incapacitated.
Furthermore, they have shared duties in maintaining the household, and in raising any children of the marriage.
Typically, creating a valid marriage is a relatively simple matter of going to a local courthouse and filing out some paperwork – with some states requiring a waiting period. Usually, the marriage must also be solemnized by a third party (usually, but not necessarily, a minister of some religion) in the presence of witnesses.
In some states, there is another way to create a marriage, however:
Common-law marriage is a legally-binding marriage that arises not out of a formal agreement but out of custom and habit. If two people live together for many years, and behave as husband and wife, while holding themselves out to the public as such, the law will recognize them as being married, and create all of the legal rights and obligations associated with marriage. It is only recognized in a small minority of U.S. states. While it was once prevalent in the U.S., most states have done away with it in favor of the certainty that comes with only recognizing formal marriages.
For a common-law marriage to come into being, the parties must behave exactly as if they were formally married, including using the same last name, filing joint tax returns, etc.
Of course, there are no "marriage police" who can come and tell you the exact moment your relationship has turned into a common-law marriage. And most couples who have lived together that long without getting a marriage certificate probably don't really care if the state recognizes their union as a formal marriage or not. This is all well and good…until whether or not the couple was actually married becomes a contested issue.
This often becomes an issue after one of the "spouses" has died. For example, if a person dies without a will, their property will automatically go to their closest living relative. The closest relative a person can have, for this purpose, is a spouse. If they were in a common-law marriage, the surviving "spouse" will have to prove that they were, in fact, married if he or she wants to inherit. Proving this after the fact is obviously not always easy.
For that reason, it's a good idea to simply get a marriage certificate if you and your partner plan on spending the rest of your lives together. If you do not want to, or cannot, get married, you should look into other legal arrangements that will protect your rights.