Schroeder's staircase

Schroeder's staircase 

his is an ambiguous figure that gives rise to two different perceptions. It consists of a drawing of parallel step-like lines extending from the upper corner of a parallelogram to the lower opposite corner. One either sees a staircase from underneath or a staircase from above and, upon continuous viewing, the impression alternates (Fig. S2). Syn. Schroeder's staircase visual illusion. See Blivet figure; Necker cube; Rubin's vase.
Fig. S2 Schroeders staircaseenlarge picture
Fig. S2 Schroeder's staircase