At one end it looks like three round columns. At the other end it looks like two flat prongs. When you look at the Impossible Trident you switch back and forth, seeing it first as one and then as the other.
There is no way to reconcile the two shapes, and yet they are presented as one. The presentation looks logical; the mind wants to maintain that sense of logic.
Escher’s Doric Columns play with that desire to make a logical whole out of different parts. At one glance the objects in it look like two rounded, weighty, solid columns. Look again and you see shapes that fold over and under and through the columns, curving like ribbons.
If you look closely you can see a line that divides runs diagonally across the surface, dividing the image. On each side of the line you can see the mass of a column and the curving flow of the shape that connects it to the other side of the image.
Escher made that shape by taking a length of the column he had drawn and cutting it out of the picture, flattening and folding and shaping it, then putting it back into the picture.
He left us with a puzzle. Three dimensional column or flat shape – how do we see them and can we reconcile them? It’s another illusion.
M. C. Escher, Doric Columns