Ever wonder what happens to your mortgage after you close on a house? It doesn’t just sit in a vault. Thousands of loans are bundled together and turned into bonds that investors all over the world buy and sell every day. That transformation is the world of mortgage-backed securities — a $10‑trillion‑plus market that shapes the interest rates you pay and the returns pensions and mutual funds earn. In this chapter we’ll walk through how these securities are built, why their cash flows are more complicated than ordinary bonds, and how smart design creates “slices” that appeal to very different appetites for risk.