A rate "lock" or "commitment" is a lender's promise to lock in a specific interest rate and a certain number of points for you for a certain period of time while your application is processed. This ensures that your interest rate will not rise as you are working through the application process.
Rate lock periods can be various lengths of time, anywhere from 15 to 60 days, with the longer spans usually costing more. A lender will agree to freeze an interest rate and points for a longer period, such as 60 days, but in exchange, the rate (and sometimes points) will be more than with a rate lock of a shorter period.
There are other ways to get a better rate, in addition to choosing a shorter rate lock period. A bigger down payment will get you a better interest rate, because you'll be starting out with a good deal of equity. You could opt to pay points to reduce your rate over the life of the loan, meaning you pay more up front. One strategy that makes financial sense for many people is to pay points to improve the rate over the life of the loan. You'll pay more initially, but you'll save money in the end.
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