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 will not get higher while you are going through the application process.
Although there are several lengths of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. A lender can agree to freeze an interest rate and points for a longer span of time, like 60 days, but in exchange, the rate (and sometimes points) will be higher than that of a rate lock of a shorter period.
In addition to going with the shorter rate lock period, there are more ways you can attain the lowest rate. A bigger down payment will give you a better interest rate, because you'll have more equity at the start. You might opt to pay points to lower your rate over the loan term, meaning you pay more initially. For a lot of people, this is a good option..
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