When you are offered a "rate lock" from your lender, it means that you are guaranteed to get a set interest rate for a determined period while you work on the application process. This prevents you from going through your whole application process and finding out at the end that your interest rate has risen higher.
Rate lock periods can be various lengths of time, between 15 to 60 days, with the longer spans generally costing more. A lending institution can agree to hold an interest rate and points for a longer period, say 60 days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of fewer days.
In addition to opting for the shorter lock period, there are other ways you may be able to attain the lowest rate. A bigger down payment will result in a reduced interest rate, because you'll have a good amount of equity from the beginning. You can pay points to lower your interest rate for the life of the loan, meaning you pay more up front. To many people, this makes financial sense..
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