A rate "lock" or "commitment" is a promise from the lender to lock in a particular interest rate and a specific number of points for you for a certain period while your application is processed. This prevents you from working through your entire application process and finding out at the end that your interest rate has gone up.
Rate lock periods can be various lengths of time, anywhere from fifteen to sixty days, with the longer spans usually costing more. You can get a longer period for your lock, but in doing so, will probably have a higher interest rate than you would have with a shorter rate lock span of time
There are other ways to get a low rate, besides opting for a shorter rate lock period. A larger down payment will get you a better interest rate, because you will have more equity from the beginning. You can pay points to reduce your interest rate over the life of the loan, meaning you pay more up front. To a lot of people, this makes financial sense..
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