A rate "lock" or "commitment" is a promise from the lender to set a particular 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 rise while you are working through the application process.
Rate lock periods can be various lengths of time, between 15 to 60 days, with the longer period generally costing more. You can get a longer period for your lock, but in choosing this option, will probably have a higher interest rate than you would with a shorter period
In addition to going with the shorter lock period, there are other ways you can attain the lowest rate. The bigger the down payment, the better the rate will be, as you will have more equity from the start. You might opt to pay points to bring down your interest rate for the life of the loan, meaning you pay more up front. To a lot of people, this makes financial sense..
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