A rate "lock" or "commitment" is a lender's promise to lock in a specific interest rate and a certain number of points for you for a certain period of time while your application is processed. This means your interest rate will not go up during the application process.
While there may be a choice of rate lock periods (from 15 to 60 days), the longer ones are typically more expensive. The lender will agree to hold an interest rate and points for a longer period, say sixty days, but in exchange, the rate (and sometimes points) will be higher than that of a rate lock of a shorter period.
There are other ways to get a low rate, in addition to going with a shorter rate lock period. The larger the down payment, the smaller the rate will be, as you will be starting with more equity. You can pay points to lower your rate for the loan term, meaning you pay more initially. One strategy that is a good option for some is to pay points to bring the rate down over the life of the loan. You'll pay more up front, but you will save money in the end.
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