A rate "lock" or "commitment" is a promise from the lender to freeze a specific interest rate and a certain number of points for you for a certain period of time while your application is processed. This saves you from going through your entire application process and learning at the end that your interest rate has gotten higher.
Although there can be a choice of rate lock periods (from 15 to 60 days), the longer ones are generally more expensive. The lending institution will agree to hold 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 that of a rate lock of fewer days.
In addition to choosing the shorter lock period, there are other ways you can attain the best rate. The larger down payment you pay, the better your interest rate will be, since you will be starting with more equity. You can pay points to lower your rate for the loan term, meaning you pay more up front. For a lot of people, this makes financial sense..
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