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 while your application is processed. This keeps you from getting through your whole application process and discovering at the end that the interest rate has gone up.
Although there can be a choice of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. A lender will agree to lock in an interest rate and points for a longer period, such as sixty days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of a shorter period.
There are more ways to get a low rate, besides agreeing to a shorter rate lock period. The bigger the down payment, the better the rate will be, since you will be entering the loan with more equity. You could opt to pay points to improve your interest rate for the life of the loan, meaning you pay more up front. One strategy that is a good option for many people is to pay points to improve the interest rate over the term of the loan. You are paying more up front, but you will come out ahead in the end.
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