**The drag mode**

The most prominent feature of "dynamic geometry systems" is the "drag mode". The following construction shows what is meant by this. The drawing shows a triangle together with its circumcircle and its incircle. The details of the construction are hidden:

To **"drag"** the drawing, just do the following:

- Move the mouse to one of the points A, B or C.
- When the mouse cursor changes to a pair of pliers press the left mouse button and hold it down.
- Move the mouse!

Whenever you move one of the points A, B or C, the whole drawing is immediately updated as if it had been constructed with the dragged point in the new position. So the **geometric dependencies** between the objects of the drawing are preserved: the circumcircle always runs through A, B and C, and the incircle always has the triangles' sides as tangent lines, no matter where you drag these points to!

Such a **dynamic drawing** is a much better representation of a "construction" as a single static picture could ever be, because the dynamic drawing allows to browse through a great number of examples that belong to the same construction plan. So the dynamic drawing is in a certain sense a whole **class of pictures**.