30 year fixed mortgages in the US typically allow prepayment with no penalty other than some relatively nominal origination costs on the new loan, so you have a

30 year fixed mortgages in the US typically allow prepayment with no penalty other than some relatively nominal origination costs on the new loan, so you have a free option to lower by taking out a new mortgage when rates drop, while being protected on the other side. That combined with a few other features make them an extraordinarily good deal (especially when they have a strongly negative real rate, like now).