1. A printing machine for printing security features onto a web, the machine comprising: a first printing cylinder defining a first circumference and carrying a first number (m) of rows of first image elements, each adjacent row of first image elements having a first circumferential separation, wherein each row of first image elements on the first cylinder is different to all other rows on the first cylinder; a second printing cylinder defining a second circumference, different to the first circumference, and carrying a second number (n) of rows of second image elements, different to the first number (m) of rows, where each adjacent row of second image elements has the same first circumferential separation as adjacent rows of first image elements, wherein each row of second image elements on the second cylinder is different to all other rows on the second cylinder; and a third printing cylinder defining a third circumference, different to the first circumference and the second circumference, and carrying a third number (p) of rows of third image elements, different to the first number (m) of rows and different to the second number (n) of rows, where each adjacent row of third image elements has the same first circumferential separation as adjacent rows of first image elements and as adjacent rows of second image elements, so that a repeat row does not occur more frequently than once in every fourth number (q) of rows, where the fourth number (q) is less than or equal to the first number (m) times the second number (n); whereby the printing cylinders are operable to print the first and second image elements in registration on a web so that composite security features are created on the web from the first and second image elements.