A rate "lock" or "commitment" is a promise from the lender to lock in a specific interest rate and a specific number of points for you for a specified period of time while your application is processed. This keeps you from working through your entire application process and discovering at the end that the interest rate has risen higher.
While there may be a choice of rate lock periods (from 15 to 60 days), the extended ones are usually more expensive. You can get a longer period for your lock, but in doing so, will likely have a higher rate than you would with a shorter span of time
In addition to opting for the shorter rate lock period, there are other ways you may be able to attain the best rate. A bigger down payment will get you a better interest rate, since you'll have a good deal of equity at the start. You could choose to pay points to bring down your rate over the life of the loan, meaning you pay more up front. For many people, this is a good option..
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