Nephroid

c : fixed circle,
d : fixed length,
B : fixed point.
A : point moving on circle c.

On BA draw AC = d. From C draw CD = d, cutting the line OA at D.
As A moves on the circle, point D describes the curve shown (red).

Masters that modify the geometric locus: d, B, c
Operating with the selection-tool (CTRL+1 to activate it), catch and modify any of the above three objects,
to see how the locus changes.

To see the locus generation in slow motion switch to the pick-move tool (CTRL+2 to activate it), pick and move point A on circle c.

http://users.math.uoc.gr/~pamfilos/