A rate "lock" or "commitment" is a lender's promise to lock in a particular interest rate and a particular number of points for you for a specified period of time during your application process. This means your interest rate won't get higher during the application process.
While there are several lengths of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. A lender will agree to hold an interest rate and points for a longer span of time, like 60 days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of fewer days.
In addition to choosing the shorter rate lock period, there are several ways you can get the best rate. A bigger down payment will give you a better interest rate, since you'll have more equity from the beginning. You may opt to pay points to reduce your interest rate over the life of the loan, meaning you pay more up front. For many people, this makes financial sense..
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