A rate "lock" or "commitment" is a promise from the lender to lock in a certain interest rate and a specific number of points for you for a specified period of time during your application process. This ensures that your interest rate can't rise during the application process.
Although there are several lengths of rate lock periods (from 15 to 60 days), the extended ones are typically more expensive. A lending institution may agree to lock in an interest rate and points for a longer span of time, say sixty days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of a shorter period.
In addition to choosing a shorter lock period, there are several ways you can score the lowest rate. The bigger down payment you pay, the better the interest rate will be, as you will have more equity from the beginning. You can pay points to bring down your rate over the term of the loan, meaning you pay more up front. One strategy that makes financial sense for some is to pay points to improve the rate over the term of the loan. You'll pay more up front, but you will save money, especially if you don't refinance early.
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