When a credit card company sends out a bill there is time between when it is sent and when the bill is paid. The time in between is when residual interest is applied. It is only used when there is a balance is left on the bill.
A customer needs to pay off the total of the bill the day the bill is sent from the company to not incur residual interest. This sometimes throws people off because they will have a bill the month after they paid off their bill because of residual interest.
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