In the business world, cash does not always change hands immediately upon the transfer of the accompanying goods or services. Most businesses must often sell their products to customers on credit -thus, the product is sold and an invoice is sent to the customer, and the customer pays the purchase price within a set time period. Using this model, there is an inherent risk of customer default. These defaults must be appropriately recognized as expenses by the seller, and an allowance must be maintained for the default of these so-called "doubtful" accounts. Learning how to account for doubtful debts is a matter of recording the allowance properly.