Supervision can also provide control in systems that are properly segregated. For example, the mail room is a point of risk in most cash receipts systems. The individual who opens the mail has access both to cash (the asset) and to the remittance advice (the record of the transaction). A dishonest employee may use this opportunity to steal the check, cash it, and destroy the remittance advice, thus leaving no evidence of the transaction. Ultimately, this sort of fraud will come to light when the customer complains after being billed again for the same item and produces the canceled check to prove that payment was made. By the time the firm gets to the bottom of this problem, however, the perpetrator may have committed the crime many times and left the organization. Detecting crimes after the fact accomplishes little; prevention is the best solution. The deterrent effect of supervision can provide an effective preventive control.