So long as a business pays on delivery for everything it buys the purchases account or accounts could be written up from the entries in the cash paid book or the cash paid side of a single cash book. Where purchases are obtained on credit it is necessary to operate a separate book of prime entry recording such transactions. This subsidiary book is variously called a purchase journal or purchase day book.

The purpose of the purchase journal is to provide a medium from which entries can be made :
a. To the creditor’s accounts recording the indebtedness of the business to its suppliers
b. To the purchases or material account, recording the cost of the material which has been obtained but not paid for immediately.