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; and 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; 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.