Blender 3D: Noob to Pro/Orthographic Views

Orthographic Views
An view (or projection) of a 3D scene is a 2D picture of it in which parallel lines appear parallel, and all edges perpendicular to the view direction appear in proportion, at exactly the same scale.

Orthographic views are usually aligned with the scene's primary axes. Edges parallel to the view axis disappear. Those parallel to the other primary axes appear horizontal or vertical. The commonly used orthographic views are front, side, and top views, though back and bottom views are possible.

Uniform scale makes an orthographic view very useful when constructing 3D objects, not only in computer graphics, but also in manufacturing and architecture.

Here's one way to think about the orthographic view:

Imagine photographing a small 3D object through a telescope from a very great distance. There would be no foreshortening. All features would be at the same scale, regardless of whether they were on the near side of the object or its far side. Given two (or preferably three) such views, along different axes, you could get an accurate idea of the shape of the object, useful for "getting the feel" of objects in a virtual 3D world where you're unable to touch or handle anything!

Example
Here is a drawing of a staircase:



and here are three orthographic views of the same staircase, each outlined in red:



The views are from the front, top, and left. Dashed lines represent edges that, in real life, would be hidden behind something, such as the left wall of the staircase. (Think of each view as an X-ray image.)

The leading edges of the steps are visible in both the front and top views. Note that they appear parallel and of equal length in 2D, just as they are in 3D reality.