A rate "lock" or "commitment" is a promise from the lender to lock in a specific interest rate and a certain number of points for you for a certain period during your application process. This means your interest rate can't go up during the application process.
While there can be a choice of rate lock periods (from 15 to 60 days), the extended spans are usually more expensive. The lending institution can agree to hold an interest rate and points for a longer span of time, like 60 days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of fewer days.
In addition to going with the shorter lock period, there are more ways you may be able to get the lowest rate. A bigger down payment will result in a reduced interest rate, since you are starting out with more equity. You might choose to pay points to reduce 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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