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A rate "lock" or "commitment" is a promise from the lender to hold a particular interest rate and a certain number of points for you for a certain period of time during your application process. This keeps you from getting through your whole application process and finding out at the end that the interest rate has gone up.
Although there are various lengths of rate lock periods (from 15 to 60 days), the longer spans are generally more expensive. A lender may agree to freeze an interest rate and points for a longer span of time, like 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 the shorter rate lock period, there are several ways you are able to score the best rate. A bigger down payment will give you a better interest rate, since you will be starting out with a good deal of equity. You may opt to pay points to improve your rate over the life of the loan, meaning you pay more initially. One strategy that is a good option for many people 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 in the long run.
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