A rate "lock" or "commitment" is a lender's promise to hold a particular interest rate and a particular number of points for you for a certain period of time during your application process. This prevents you from working through your whole application process and learning at the end that your interest rate has gotten higher.
Rate lock periods can vary in length, anywhere from 15 to 60 days, with the longer spans usually costing more. A lending institution may agree to hold an interest rate and points for a longer period, like 60 days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of fewer days.
In addition to going with the shorter lock period, there are other ways you may be able to attain the lowest rate. A larger down payment will result in a reduced interest rate, because you'll have a good deal of equity at the start. You may opt to pay points to lower your interest rate for the loan term, meaning you pay more initially. One strategy that makes financial sense for many people is to pay points to improve the rate over the term of the loan. You'll pay more up front, but you'll save money in the long run.
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