When you're promised a "rate lock" from your lender, it means that you are guaranteed to keep a particular interest rate for a determined period while you work on the application process. This ensures that your interest rate will not get higher during the application process.
Rate lock periods can vary in length, between 15 to 60 days, with the longer ones typically costing more. A lender may agree to hold an interest rate and points for a longer period, like 60 days, but in exchange, the rate (and sometimes points) will be higher than that of a rate lock of fewer days.
There are other ways to get a low rate, in addition to choosing a shorter rate lock period. A bigger down payment will give you a lower interest rate, because you will be starting out with a good deal of equity. You can pay points to lower your interest rate for the term of the loan, meaning you pay more up front. To many people, this makes financial sense..
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