A rate "lock" or "commitment" is a promise from the lender to set a certain interest rate and a certain number of points for you for a certain period during your application process. This keeps you from getting through your whole application process and learning at the end that your interest rate has gotten higher.
Although there might be a choice of rate lock periods (from 15 to 60 days), the extended spans are generally more expensive. The lending institution can agree to lock in an interest rate and points for a longer period, like sixty days, but in exchange, the rate (and sometimes points) will be more than with a rate lock of a shorter period.
There are more ways to get a reduced rate, in addition to going with a shorter rate lock period. A larger down payment will get you a lower interest rate, since you'll have a good amount of equity at the start. You might choose to pay points to reduce your interest rate for the life of the loan, meaning you pay more up front. For many people, this makes sense and is a good deal..
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