Things, however, can go wrong. People get laid off, sick, and divorced. That promotion or raise you've been counting on fails to come through, and the cost of living climbs higher than your income. They will wind up paying it off in 20 or 25 years, whatever, without being trapped by sickness or a bad year. One option is to make a 13th payment every year.
Some people make an actual extra payment every year.
Others increase the size of their monthly mortgage check by one-twelfth every month so that it adds up to 13 payments at the end of a year. Still others make half a mortgage payment every two weeks, so at the end of the 52-week year they have made 26 half payments or 13 whole ones. That's one way to do it. That way you know what you have to pay in order to do it, and if your finances force you to fall behind you'll be okay as long as you don't fall behind on the regular 30-year payment. Even if you do fall behind on your accelerated payment, every extra payment you make does pay off in the end.