A rate "lock" or "commitment" is a lender's promise to set a specific interest rate and a certain number of points for you for a certain period of time while your application is processed. This protects you from getting through your entire application process and discovering at the end that the interest rate has gone up.
Although there can be a choice of rate lock periods (from 15 to 60 days), the longer spans are generally more expensive. The lending institution will agree to hold an interest rate and points for a longer span of time, such as 60 days, but in exchange, the rate (and sometimes points) will be higher than that of a rate lock of fewer days.
There are other ways to get a low rate, in addition to opting for a shorter rate lock period. The larger down payment you pay, the smaller your interest rate will be, as you will be starting with more equity. You may choose to pay points to reduce your interest rate over the life of the loan, meaning you pay more initially. One strategy that makes financial sense for many people is to pay points to bring the rate down over the life of the loan. You'll pay more up front, but you'll save money, especially if you keep the loan for the full term.
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