When you're promised a "rate lock" from the lender, it means that you are guaranteed to get a set interest rate for a certain number of days for the application process. This protects you from working through your entire application process and learning at the end that the interest rate has gone up.
Rate lock periods can be various lengths of time, between fifteen to sixty days, with the longer ones usually costing more. The lending institution 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 more than that of a rate lock of a shorter period.
In addition to choosing a shorter rate lock period, there are other ways you can get the lowest rate. The more the down payment, the smaller the interest rate will be, since you will have more equity from the beginning. You can pay points to improve your interest rate over the life of the loan, meaning you pay more initially. To a lot of people, this is a good option..
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