Model for Descriptive Geometry by A. Jullien - Representation of a Point

Jullien, A. · ca 1880

For the discussion that follows, the following conventions will be used to explain the location of points in each model. Any point in space can be denoted by a coordinate triple (x,y,z). This is the three-dimensional version of the (x,y) plane from Euclidian geometry learned in high school. For our purposes, the x-axis will be the horizontal line stretching left and right at the fold of each relief. The y-axis will be on the horizontal plane (paper card) of each model, appearing to be coming out of the plane of each image. The z-axis will be the vertical axis of each relief and lies on the vertical plane (paper card) of each relief. In all of the reliefs, the x-coordinate is irrelevant since each projection will be with respect to the y and z planes. When needed, a point will be referred to in two coordinates only (y,z), leaving off the x-coordinate for brevity. Positive will be the forward or upward direction and negative values will be behind or below the cards of each relief.