The upper side of the left-lower triangle is actually aligned with the upper side of the right-upper triangle, the former appears to tilt counterclockwise while the latter clockwise.
Bourdon, B. (1902) La perception visuelle de l'espace. Paris: Reinwald.
In my vision, anomalous motion illusion is also observed in the same direction as the tilt illusion. <September 3, 2005>
Catalogue of illusions