You might have noticed that there are two circles which each describe a position of one of the rods. The configuration space might look like this
![](pic26.jpg)
EXCEPT that now two points, one from each, must be chosen to specify a configuration. We want a space where a single point corresponds to one linkage position. We need a circle's worth of circles:
![](pic27.jpg)
which will look like the surface of a donut, or a torus, as it is called. Can you see how this is a circle's worth of circles?
![](pic28.jpg)
Now a continuous motion of our linkage corresponds to a path of the torus.
![](pic29.jpg)
Let's look at one more. Consider the square linkage with two adjacent anchors:
![](pic30.jpg)
In what way can it move?