Number of Horizontal Rhombuses
How many horizontal rhombuses are there? Is there a rule for this?
A calisson is a French sweet in the shape of two equilateral triangles edge to edge. They are packed in a hexagonal box. The puzzle is to show why the number of callisons pointing in each direction, in a fully packed box, must always be equal.
 D. Wells, The Penguin Book of Curious and Interesting Puzzles, New York: Penguin Books, 1993 p. 133.