English: William Wallace's eidograph for reproducing a drawing at a different scale - Diagram explaining the mathematical principle. Adjustments to the position of pivot C along AB alter the scaling factor applied to the coordinated motion of P and T. To maintain the proper geometry, the adjusted length of AP must equal AC and BT must equal BC; further, AP and BT must remain parallel. During use, the connection parallel to AB insure that the disks A and B rotate together to keep AP and BT parallel; the connections may be realized using a cable attached to pulleys A and B or rods connected by pivots to the periphery of disks A and B. Disks A and B must be identical.