The greatest difference between drawing an outer thread and drawing an ellipse lies in the nature of the perspective presentation. Three different outer threads can be represented on the three perspective major axes.

Although the outline is the same, an outer thread with an orientation of 0° differs significantly from one with an orientation of 180°.

Six different orientations are therefore possible in theory, however in the vast majority of cases, only three are required for perspective illustration. These are pictured in the top row of the figure.

Since the outer thread shares most of the characteristics of an ellipse, the following sections only set out those areas where differences exist to the ellipse.