It is a book where petty expenses e.g postage are recorded.the only debit transaction in a petty cash book is the amount given to the cashier by the main cashier.other receipts,however small are recorded in the main cash book.
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An imprest system is where one can only reimburse the amount used e.g if at the beginning of a month there is Ksh.20,000 as pettycash float,and by the end of the month the amount spent is ksh.18,000, an amount of Ksh.18,000 will be replaced to bring the amount of petty cash float back to Ksh.20,000