When you are promised a "rate lock" from your lender, it means that you are guaranteed to keep a set interest rate for a certain number of days while you work on the application process. This ensures that your interest rate won't grow as you are working through the application process.
While there may be a choice of rate lock periods (from 15 to 60 days), the longer spans are typically more expensive. The 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 higher than with a rate lock of fewer days.
In addition to opting for the shorter rate lock period, there are more ways you are able to score the lowest rate. The more the down payment, the lower your interest rate will be, because you will have more equity from the start. You can pay points to bring down your interest rate for the loan term, meaning you pay more initially. One strategy that is a good option for some is to pay points to improve the rate over the term of the loan. You'll pay more up front, but you'll come out ahead in the end.
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