This memo explores the evolution of mortgage lending in the United States, with a particular focus on explicating the array of opaque, exotic, and increasingly complex mortgage instruments that emerged after the 1970s in tandem with the deregulation of American finance. The emergence of these mortgage types was one of many contributing factors to the housing bubble that set the stage for the 2008 financial crisis. After clarifying the most common features of subprime mortgages, the memo traces the historical evolution of American residential mortgages more generally, and exotic Adjustable Rate Mortgages in particular.