Thanks guys for all the feedback on this. I had no idea there would be so many and varied algorithms in use!
I have determined that the one used here is a very simple one that can be expressed by the Excel formula =IF(MOD(C3,11)=10,"x",MOD(C3,11)) where C3 is the base number So essentially it is just the remainder after dividing the base number by 11, with 10 being set to "x". I guess it is simple in this case because it is only testing against mistakes in keying: not trying to prevent fraudulent trying of numbers eg Credit cards Thanks again Mark _______________________________________________ Delphi mailing list [EMAIL PROTECTED] http://ns3.123.co.nz/mailman/listinfo/delphi
