A rate "lock" or "commitment" is a promise from the lender to set a certain interest rate and a particular number of points for you for a specified period while your application is processed. This protects you from getting through your entire application process and finding out at the end that your interest rate has risen higher.
Rate lock periods can vary in length, anywhere from 15 to 60 days, with the longer spans typically 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 with a shorter span of time
There are more ways to get a reduced rate, besides going with a shorter rate lock period. The larger the down payment, the lower your interest rate will be, since you will have more equity from the start. You can pay points to bring down your rate over the term of the loan, meaning you pay more up front. To many people, this is a good option..
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