When you are promised a "rate lock" from the lender, it means that you are guaranteed to keep a certain interest rate for a certain number of days while you work on your application process. This means your interest rate won't go up during the application process.
While there can be a choice of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. You can get a longer period for your lock, but in doing so, will probably have a higher interest rate than you would have with a shorter rate lock span of time
There are more ways to get a good rate, besides opting for a shorter rate lock period. A bigger down payment will get you a reduced interest rate, since you will have more equity from the beginning. You can pay points to lower your rate for the term of the loan, meaning you pay more up front. For many people, this is a good option..
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