<prompt> Human: Might be a dumb question, but Im looking to buy, and I always wondered about thatIs it just because 20 is a clean number? But you could say 25 is also clean :  <response>: During the Great Depression, every private mortgage insurer went bust. After that, there was only federal mortgage insurance until 1957, when the Mortgage Guaranty Insurance Corporation was founded. To ensure that the same thing didnt happen again, MGIC would only insure the first 20 of the house. As a result, banks started requiring 20 or you pay PMI, to completely cover their investment unless the house takes a big hit in value, of course. 