(57) This invention relates to documents, such as banknotes, which are produced and used
in large numbers and which, for a given series, are identical one with another except
for a unique multi-digit number upon each individual document, the number thus uniquely
identifying that document within the series. According to the present invention, to
render unauthorised reproduction of the document more difficult, on each document
the numerical or alphabetical characters in the identifying number are provided one
above the other. In addition, at least two characters in the identifying number may
differ from each other (in addition to any differences in the letters or digits which
they represent) in one or more visible characteristics. Thus the differing characters
may have different heights or widths or be in different styles of type. Preferably,
the identifying number appears twice on each document, in diagonally opposite corners,
and preferably the variation of physical characteristics of the characters along the
length of one of the numbers is reversed in direction in the other of the numbers.
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