A rate "lock" or "commitment" is a promise from the lender to lock in a particular interest rate and a certain number of points for you for a specified period during your application process. This saves you from going through your whole application process and discovering at the end that your interest rate has gotten higher.
While there can be a choice of rate lock periods (from 15 to 60 days), the longer ones are generally more expensive. You can get a longer period for your lock, but in making this choice, will likely have a higher interest rate than you would have with a shorter period
There are other ways to get a lower rate, besides opting for a shorter rate lock period. The larger down payment you can pay, the lower your interest rate will be, since you will have more equity from the start. You could opt to pay points to improve your interest rate over the life of the loan, meaning you pay more initially. For a lot of people, this makes financial sense..
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