A rate "lock" or "commitment" is a lender's promise to freeze a certain interest rate and a particular number of points for you for a certain period during your application process. This means your interest rate won't grow during the application process.
Rate lock periods can be various lengths of time, between fifteen to sixty days, with the longer spans generally costing more. You can get a longer period for your lock, but in choosing this option, will probably have a higher rate than you would with a shorter rate lock period
There are more ways to get a lower rate, besides going with a shorter rate lock period. The larger the down payment, the lower the rate will be, since you will be starting with more equity. You might opt to pay points to bring down your rate over the life of the loan, meaning you pay more up front. For a lot of people, this is a good option..
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