When you are promised a "rate lock" from your lender, it means that you are guaranteed to get a certain interest rate for a certain number of days for your application process. This means your interest rate can't rise while you are working through the application process.
Rate lock periods can be various lengths of time, anywhere from 15 to 60 days, with the longer spans typically costing more. A lending institution may agree to hold an interest rate and points for a longer span of time, such as sixty days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of fewer days.
There are other ways to get a lower rate, besides going with a shorter rate lock period. A larger down payment will result in a reduced interest rate, since you will have a good deal of equity from the beginning. You can pay points to reduce your interest rate over the term of the loan, 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 will pay more up front, but you'll come out ahead, especially if you don't refinance early.
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