What is the difference between a marriage void ab initio and a voidable nunc pro tunc?

In general, a marriage void ab initio is one where 1) one spouse was bigamous or polygamous at the time of the marriage in question; or 2) the parties are related to "a degree of affinity prohibited by law, i.e., ancestors (parents, grandparents of any degree), descendants (children, grandchildren of any degree), siblings, uncles, aunts, nieces, nephews, or first and, in many jurisdictions, second cousins"; or 3) in some states, one of the parties was under the jurisdiction’s minimum age, which is usually 15 or 16.

A marriage is voidable nunc pro tunc under a variety of circumstances, including, but not limited to, marriages where one spouse 1) misrepresented the desire to have, or not to have, children; or 2) misrepresented "the intention (whether implicit or explicit) to consummate the marriage -- and did not do so -- ever"; or 3) misrepresented his or her health or legal status, including sexual diseases, fabricated pregnancy, criminal convictions, prior marriages; or 4) is of the same sex; or 5) is of impaired capacity.

In general, in marriages voidable nunc pro tunc the successful plaintiff must allege that he or she would not have "entered in the marriage if the truth were known."

A party tricked into marriage by, for example, a fabricated pregnancy may be able to obtain an annulment if the facts favor that decision. An annulment is not a substitute for divorce, but is instead an entirely separate and different procedure.