The use of thick and thin lines is an important stylistic device in Technical Illustration.

A technical illustration allows the illustrator to depict technical coherences and descriptions by relatively simple means. It deliberately dispenses with extraneous details and graphic information.

At the same time, however, the viewer should be able to quickly grasp the situation shown in the graphic. This is realized by applying thick and thin lines to the object depicted. The following graphics visualize this effect:


 
As the left graphic comes with different line weights, the object is more clearly depicted than in the right illustration. The thick lines are applied where a surface borders on another, invisible surface. If you can see both adjoining surfaces, the border line must be represented by a thin line.

As an auxiliary method, you might also imagine reaching behind a line with your hand: If you are not able to see your fingertips, then the line must be thick:

Further examples of technical illustrations can be found in the Gallery.