When you're promised a "rate lock" from the lender, it means that you are guaranteed to keep a particular interest rate for a determined period for your application process. This ensures that your interest rate won't grow during the application process.
While there may be a choice of rate lock periods (from 15 to 60 days), the longer ones are usually more expensive. A lender may agree to freeze 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 better rate, besides agreeing to a shorter rate lock period. The more the down payment, the smaller your rate will be, as you will be starting with more equity. You could opt to pay points to improve your interest rate for the loan term, meaning you pay more initially. One strategy that makes financial sense for some is to pay points to bring the rate down over the life of the loan. You pay more initially, but you'll come out ahead, especially if you don't refinance early.
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