When you are promised a "rate lock" from your lender, it means that you are guaranteed to get a certain interest rate over a determined period for your application process. This means your interest rate won't rise while you are going through the application process.
While there might be a choice of rate lock periods (from 15 to 60 days), the longer ones are typically more expensive. A lender can agree to hold an interest rate for a longer period, say sixty days, but in exchange, the rate will be more than that of a rate lock of fewer days.
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