When you are promised a "rate lock" from the lender, it means that you are guaranteed to keep a particular interest rate for a certain number of days while you work on the application process. This means your interest rate cannot rise as you are going through the application process.
While there might be a choice of rate lock periods (from 15 to 60 days), the extended spans are generally more expensive. You can get a longer period for your lock, but in making this choice, will likely have a higher rate than you would have with a shorter rate lock period
There are more ways to get a lower rate, besides opting for a shorter rate lock period. The bigger the down payment, the lower the rate will be, as you will be entering the loan with more equity. You can pay points to lower your interest rate over the term of the loan, meaning you pay more up front. For many people, this makes financial sense..
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