When you're promised a "rate lock" from a lender, it means that you are guaranteed to keep a set interest rate for a determined period while you work on the application process. This means your interest rate will not go up during the application process.
Rate lock periods can be various lengths of time, anywhere from fifteen to sixty days, with the longer period usually costing more. The lender can agree to hold an interest rate and points for a longer span of time, say 60 days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of a shorter period.
In addition to going with a shorter lock period, there are other ways you are able to get the lowest rate. A bigger down payment will give you a reduced interest rate, since you will be starting out with more equity. You could opt to pay points to improve your rate over the life of the loan, meaning you pay more up front. For many people, this makes financial sense..
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