A rate "lock" or "commitment" is a promise from the lender to freeze a certain interest rate and a certain number of points for you for a specified period of time during your application process. This protects you from going through your whole application process and learning at the end that your interest rate has risen higher.
While there are several lengths of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. The lending institution can agree to hold an interest rate and points for a longer span of time, such as sixty days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of a shorter period.
There are more ways to get a better rate, besides choosing a shorter rate lock period. A bigger down payment will give you a reduced interest rate, because you are starting out with more equity. You can pay points to bring down your interest rate for the loan term, meaning you pay more up front. To many people, this makes financial sense..
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