A rate "lock" or "commitment" is a promise from the lender to hold a specific interest rate and a particular number of points for you for a certain period of time while your application is processed. This prevents you from getting through your whole application process and learning at the end that the interest rate has gone up.
While there are several lengths of rate lock periods (from 15 to 60 days), the extended spans are generally more expensive. You can get a longer period for your lock, but in making this choice, will likely have a higher rate than you would have with a shorter rate lock period
In addition to choosing the shorter lock period, there are several ways you can attain the lowest rate. A larger down payment will give you a better interest rate, since you will have more equity at the start. You could choose to pay points to improve your rate over the loan term, meaning you pay more initially. To a lot of people, this makes financial sense..
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