When you are promised a "rate lock" from a lender, it means that you are guaranteed to keep a certain interest rate for a determined period for the application process. This prevents you from getting through your whole application process and discovering at the end that the interest rate has gone up.
While there are several lengths of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. The lender will 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 more than that of a rate lock of a shorter period.
There are more ways to get a reduced rate, besides going with a shorter rate lock period. A larger down payment will give you a better interest rate, since you'll be starting out with a good deal of equity. You could choose to pay points to improve your interest rate over the term of the loan, meaning you pay more initially. One strategy that is a good option for some is to pay points to improve the interest rate over the term of the loan. You'll pay more initially, but you will save money, especially if you don't refinance early.
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