Under Generally Accepted accounting Principles, WorldCom was required to estimate its line costs each month and to expense the estimated cost immediately, even though many of these costs would be paid later. To reflect an estimate of amounts that had not yet been paid, WorldCom would set up a liability account, know as an accrual, on its balance sheet. As the bills arrived from its outside parties,sometimes many month later, WorldCom would pay them and reduce the previously established accruals accordingly.