It's a legal technicality that might buy you some time if you are in foreclosure. Basically it involves insisting that the party that initiated the foreclosure prove that they have the right to payments under the promissory note, which is part of your mortgage. It works better in jurisdictions that require judicial foreclosure because in a state that permits non-judicial foreclosure, you would need to start a legal action yourself in order to prevent the foreclosure from continuing. The defense is possible because of the way the secondary mortgage market works. Mortgages are passed around a lot. Sometimes the rights to the mortgage are sold and the mortgage documents are kept by a custodian or sent along later. They can be lost or misplaced. Promissory notes are designed to be passed around like this, but there are some technical requirements for doing so. In most cases the lenders can fix whatever mistakes they have made and produce the note (notes are usually imaged now anyway), but demanding the note can buy you some time. While all of this is true, it's not safe to assume that sending a letter asking for the note will stop any foreclosure. Legal requirements vary quite a bit among jurisdictions. You should talk to a lawyer who specializes in foreclosure defense if you want to take advantage of this defense.