The image is a photograph of an open book with a diagram on the right side. The diagram is a cross-section of a prism with a point at the center and a line at the top. The line is labeled with the letters "Z" and "C" and is drawn in black ink. On the left side of the image, there is a text that explains how to measure the angle of the prism. The text explains that the prism is made up of two angles, Z and C, and that it is the same direction as the other angles. The image also mentions that the same information is sometimes written in the same text as the diagram.