In the following drawing, the **orthocentre H** of a tringle ABC is constructed. The triangle's vertex C is bound to a circle k. When you run C along its "carrier line" k, then the point H runs on a line that is completely determined by the positions of A, B and k. The drawing holds a picture of this line; it is called a **locus line of the point H**.

When you drag C along k this has no effect on the locus line but only on the actual position of the orthocentre H. But you can also drag A or B to a new position, or make changes to the centre and radius of k. **In every case** not only the triangle ABC and its orthocentre are updated correctly, but furthermore, the locus line of H is recalculated and updated, too!

But, this drawing also shows the