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Is there any circumstance in which it is necessary to mark extra payments on a loan as going to “principal and not interest”?
The mortgage I got last year through Wells Fargo explicitly indicates in its terms that excess payment will be considered against future payments (i.e., pay $500 extra in January and you owe $500 less in February) unless indicated otherwise. It goes on to state that with electronic payments you do not get to specify where excess payment goes, so excess payment made electronically always goes toward future payments. If you want to make excess payments toward principal, you must actually send them a check and your payment stub, with the appropriate box ticked.  This won't be very different for other major banks, I wouldn't imagine.