When you're offered a "rate lock" from the lender, it means that you are guaranteed to get a certain interest rate over a determined period while you work on your application process. This prevents you from getting through your entire application process and discovering at the end that the interest rate has risen higher.
Rate lock periods can vary in length, anywhere from fifteen to sixty days, with the longer ones usually costing more. A lender can 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 more than with a rate lock of a shorter period.
There are other ways to get a better rate, besides agreeing to a shorter rate lock period. A larger down payment will get you a lower interest rate, because you are starting out with more equity. You can pay points to lower your rate for the loan term, meaning you pay more initially. One strategy that is a good option for some is to pay points to bring the rate down over the life of the loan. You'll pay more initially, but you will save money in the end.
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