When you are promised a "rate lock" from the lender, it means that you are guaranteed to keep a particular interest rate over a determined period for the application process. This means your interest rate can't go up during the application process.
Rate lock periods can be various lengths of time, between 15 to 60 days, with the longer spans generally costing more. A lender may agree to hold an interest rate and points for a longer span of time, say 60 days, but in exchange, the rate (and sometimes points) will be higher than that of a rate lock of a shorter period.
There are more ways to get a good rate, in addition to agreeing to a shorter rate lock period. The bigger down payment you can pay, the smaller the rate will be, because you will be entering the loan with more equity. You might choose to pay points to lower your interest rate over the term of the loan, meaning you pay more initially. To many people, this makes sense and is a good deal..
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