When you're offered a "rate lock" from your lender, it means that you are guaranteed to get a set interest rate for a certain number of days while you work on the application process. This protects you from getting through your whole application process and discovering at the end that your interest rate has gotten higher.
Although there may be a choice of rate lock periods (from 15 to 60 days), the extended ones are generally more expensive. The lender may agree to hold an interest rate and points for a longer span of time, like sixty days, but in exchange, the rate (and sometimes points) will be more than with a rate lock of fewer days.
In addition to opting for a shorter rate lock period, there are other ways you can attain the lowest rate. A bigger down payment will result in a lower interest rate, since you are starting out with a good deal of equity. You could choose to pay points to lower your interest rate for the term of the loan, meaning you pay more up front. For a lot of people, this makes financial sense..
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