A first pattern is created by providing an image and its enantiomorph on opposite sides of an axis of bilateral symmetry and selectively removing elements from the image and its enantiomorph without removing any element from both the image and its enantiomorph. A second pattern is created, being an enantiomorph of the first pattern. The patterns are rotated relative to each other about a common axis with both patterns visible to a viewer, whereby an intelligible pattern having at least one axis of bilateral symmetry is formed by a combination of the patterns at at least one position of the first pattern relative to the second pattern. At other positions the intelligible pattern is present only in an encoded fashion. The encoding is done by ensuring that elements are missing from the first pattern so that it is unable in itself to constitute an intelligible design, the missing elements being supplied when the first pattern is appropriately combined with the second pattern.