When interest rates climb past 6%, a fixed mortgage locks you into a costly decade. An ARM might be the smarter play — if you know how it works. A guide to understanding ARMs · May 2026 What is an adjustable rate mortgage? A mortgage is a loan secured against your home, repaid over a long period — typically 15 or 30 years. Every mortgage has an interest rate, which determines how much you pay on top of the principal you borrowed. With a fixed-rate mortgage, that rate never