Illustrations: Orthographic drawings

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Orthographic drawings provide a visual representation of an object from a number of different angles (at right-angles to each other). They are always drawn to scale and as two-dimensional (flat) images, and are therefore well-suited to showing measurements. An orthographic drawing often consists of three individual illustrations, showing the view of the object from the front, from the top, and from one side. In some cases, all six angles (front, back, top, bottom, right, and left) may be shown.

How these individual illustrations are positioned relative to each other depends on the angle of projection that is used. This will be either first-angle projection, or third-angle projection. In first-angle projection, the 'view from the top' of the object is positioned below the 'view from the front' of the object, and the 'view from the left' is positioned to the right of the object (and vice versa). In third-angle projection, the 'view from the top' is positioned above the 'view from the front', and the 'view from the left' is positioned to the left of the 'view from the front'. The following examples show the same object drawn in both first-angle projection and third-angle projection:

First-angle projection	Third-angle projection

Effectively, with first-angle projection, it is as if the object being illustrated is on top of a sheet of paper, and rolled against this paper to provide the views - so that when the object is rolled to the right, the leftmost side is presented. By contrast, with third-angle projection, it is as if the object is on the other side of a piece of glass, and rolled against this glass to provide the views - so that when the object is rolled to the right, the rightmost side is presented. This is easier to understand from the following diagram:

Orthographic projections: Object shown in first and third angle projections.

Many people find third-party projection easier to understand because the individual views are positioned adjacent to the sides that they are illustrating.