When you're promised a "rate lock" from a 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 means your interest rate won't rise during the application process.
Although there might be a choice of rate lock periods (from 15 to 60 days), the extended ones are typically more expensive. A lender will agree to hold an interest rate and points for a longer period, say sixty days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of fewer days.
There are other ways to get a good rate, in addition to agreeing to a shorter rate lock period. The bigger down payment you make, the better your rate will be, since you will have more equity from the start. You can pay points to reduce your interest rate for the loan term, meaning you pay more up front. One strategy that makes financial sense for some is to pay points to bring the rate down over the life of the loan. You'll pay more up front, but you'll save money, especially if you don't refinance early.
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