A rate "lock" or "commitment" is a promise from the lender to hold a specific interest rate and a particular number of points for you for a certain period of time while your application is processed. This means your interest rate won't get higher during the application process.
Rate lock periods can be various lengths of time, between 15 to 60 days, with the longer period generally costing more. A lender will agree to lock in an interest rate and points for a longer span of time, such as sixty 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 better rate, besides agreeing to a shorter rate lock period. A larger down payment will get you a better interest rate, since you will be starting out with a good deal of equity. You can pay points to lower your interest rate over the loan term, meaning you pay more initially. One strategy that is a good option for many people is to pay points to reduce the interest rate over the term of the loan. You pay more up front, but you'll save money in the end.
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