A rate "lock" or "commitment" is a lender's promise to hold a specific interest rate and a specific 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 finding out at the end that your interest rate has risen higher.
Rate lock periods can be various lengths of time, between 15 to 60 days, with the longer period usually costing more. The lending institution will agree to freeze 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 with a rate lock of fewer days.
There are more ways to get a low rate, besides going with a shorter rate lock period. A bigger down payment will result in a better interest rate, because you'll have more equity from the beginning. You can pay points to bring down your rate for the term of the loan, meaning you pay more initially. For many people, this makes financial sense..
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