A draftsman many times has the need for pictorial drawings, either to help clarify the form of an object in his own mind or to help someone with an untrained imagination visualize the object from the orthographic views. In a sense, the designer's perspective sketch performs this function. The sketch is of limited use to the carpenter, obviously, because of the foreshortening in the perspective.
By imagining a pictorial drawing with the edges of the receding surfaces not converging and sides not foreshortened, a type of pictorial is represented that can be drawn to scale and used as a supplementary view to the working drawings. The lack of perspective makes it possible to draw to scale. As a view, however, it has a distorted mechanical look and it is limited in its representation of curves and angles. The best use is as a pictorial view of a structural, or mechanical, detail that is not clear in the regular orthographic views.
The beginning of a pictorial drawing depends on the angle, or viewpoint, desired of the object. If it is to be, for example, a pictorial of a cube, it must be decided first how it is to look. Is it to be drawn as seen from a corner? Or is it to be viewed from one side? Is it to be seen from above looking down or from underneath looking up? The shape of the object and the location of details affect the choice of the direction of the view.
With an object as simple as a cube, a view from a corner and slightly above represents it with the least amount of distortion. Such is the usual direction of an isometric drawing, for it is started with an edge of the cube touching the plane of the paper, or picture plane, as it is called.
The term isometric, meaning equal measure as compared to the foreshortened distances, or unequal-measure of perspective, accurately describes its appearance. An isometric drawing has three axes to represent the principal planes. The first is a vertical line to indicate all the upright edges; second, a slanted line to the right, 30° to the horizontal, for the horizontal edges of the right plane; and third, a 30° line slanted to the left to represent the horizontal edges of planes to the left. These lines and all lines parallel to them are known as isometric lines. Conversely, lines that are not parallel to any of the three axes are nonisometric lines. Heights and distances can be measured on isometric lines but a nonisometric line cannot be drawn to scale.
Returning to the cube, if it is 2' in height and width and is to be drawn at a scale of 1" = 1'0", the isometric drawing can be