This occurs when two banknotes are identical in the denomination the prefix and the serial number but differ or resemble in one or more of the following : the signature of the governor of the bank the type of the safety thread the design of the paper or the issue date 

The cross over papers is usually scarce and remarkable because the amount issued is usually less than the usual amount issued in other denominations the collector usually collects the two papers so he can compare both therefore the price of the two cross over papers should be put for both together to make it easier for the buyer