Take any triangle with polygon vertices ,
, and . Pick a point on the side opposite , and draw a line parallel to
. Upon reaching the side at , draw the line parallel to
. Continue (left figure). Then the line
closes for any triangle. If is the midpoint of , then (right figure).

Let
be the ratio in which the sides of the reference
triangle are divided i.e., , and define . Then the coordinates of the vertices of the figure
are shown above.

The six vertexes of Thomsen's figure lie on an ellipse having the triangle centroid as its center.
The area of this ellipse is

Thomsen's figure is similar to a Tucker hexagon. While Thomsen's hexagon closes after six parallels, a Tucker hexagon closes after
alternately three parallels and three antiparallels.