Citizen Path, a company offering online guidance for problems commonly encountered in immigration to the United States, provides a sample I-751 affidavit letter on its website. An I-751 affidavit letter supports an application for removal of conditions on residence for the immigrant spouse of a U.S. citizen.
In general, according to Citizen Path, an I-751 affidavit letter offers information from the affiant, the person writing the letter, supporting the good faith marriage of the couple. This might include how the affiant knows the couple, how long he has known them and his fact-based knowledge about the nature of the relationship. Citizen Path notes that filing two such letters with an I-751 application can be very helpful. The I-751 form is available at the U.S. Citizenship and Immigration Services website.