A rate "lock" or "commitment" is a lender's promise to hold a particular interest rate and a certain number of points for you for a certain period of time during your application process. This protects you from getting through your entire application process and discovering at the end that the interest rate has gone up.
Although there are several lengths of rate lock periods (from 15 to 60 days), the longer spans are typically more expensive. You can get a longer period for your lock, but in making this choice, will most likely have a higher rate than you would with a shorter span of time
There are other ways to get a low rate, besides agreeing to a shorter rate lock period. The more the down payment, the better the interest rate will be, because you will be entering the loan with more equity. You can pay points to bring down your rate for the life of the loan, meaning you pay more up front. To a lot of people, this makes financial sense..
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