When you are promised a "rate lock" from the lender, it means that you are guaranteed to keep a set interest rate over a determined period while you work on the application process. This ensures that your interest rate will not rise during the application process.
Rate lock periods can be various lengths of time, anywhere from 15 to 60 days, with the longer ones usually costing more. A lending institution can agree to lock in an interest rate and points for a longer span of time, such as 60 days, but in exchange, the rate (and sometimes points) will be more than with 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 larger down payment will get you a lower interest rate, since you'll have more equity from the beginning. You could opt to pay points to bring down your interest rate for the loan term, meaning you pay more up front. For many people, this makes financial sense..
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