A rate "lock" or "commitment" is a lender's promise to freeze a certain interest rate and a certain number of points for you for a certain period of time during your application process. This prevents you from getting through your entire application process and finding out at the end that the interest rate has risen higher.
Rate lock periods can vary in length, between fifteen to sixty days, with the longer ones generally costing more. The lender can agree to freeze an interest rate and points for a longer period, like 60 days, but in exchange, the rate (and sometimes points) will be more than with a rate lock of fewer days.
In addition to going with the shorter lock period, there are several ways you may be able to score the best rate. The larger the down payment, the better your interest rate will be, because you will be entering the loan with more equity. You could opt to pay points to lower your interest rate over the loan term, meaning you pay more initially. One strategy that is a good option for some is to pay points to improve the interest rate over the term of the loan. You'll pay more up front, but you'll come out ahead, especially if you keep the loan for a long time.
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