A rate "lock" or "commitment" is a promise from the lender 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 working through your entire application process and finding out at the end that your interest rate has gone up.
Rate lock periods can be various lengths of time, anywhere from 15 to 60 days, with the longer spans usually costing more. You can get a longer period for your lock, but in doing so, will likely have a higher interest rate than you would with a shorter period
In addition to opting for the shorter rate lock period, there are more ways you can score the best rate. A bigger down payment will get you a lower interest rate, because you're starting out with a good deal of equity. You can pay points to lower your interest rate over the loan term, meaning you pay more up front. One strategy that makes financial sense for many people is to pay points to bring the rate down over the life of the loan. You'll pay more initially, but you'll save money, especially if you don't refinance early.
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