 While there were many variants, by the mid-20th century a popular version consisted of paperboard cards with holes punched at regular intervals along all four edges, a short distance in from the edges.The center of the card might be blank space for information to be written, or contain a pre-printed form, or contain a microform image in the case of edge-notched aperture cards.To record data, the paper stock between a hole and the nearest edge was removed by a special notching tool.