 This technique is based on the principle of aiming the central ray of the X-ray beam at 900 to an imaginary line which bisects the angle formed by the long axis of the tooth and the plane of the receptor.The image receptor is placed as close as possible to the tooth under investigation, without bending the packet.Applying the geometrical principle of similar triangles, the length of the tooth on the image will be the same as that of the actual length of the tooth in the mouth.