a common law affidavit is a written document that presents sworn statements offered under the penalties of perjury to establish the existence of a common law marriage. an affidavit is a legal document that is prepared by a witness to attest to facts of which the witness has personal knowledge. the document is signed under oath and in front of a notary or other officer of the court. it requires third parties to treat spouses as a single legal entity for purposes of taxes, benefits, or anything else that one spouse would be entitled to under the law. under ordinary circumstances, if two people are merely living together without meeting the legal formalities for a marriage, they are not married.