A triangular-wave grating is perceived to fluctuate over time: at one moment it may appear veridical (ie triangular), at another it may more closely resemble a square-wave grating with rounded edges. In addition, the square-wave illusion itself is bistable, in that it sometimes appears to shift in phase by 180 deg. Experiments in which the phase and amplitude of the first three frequency components of the triangular-wave grating were independently varied showed that the square-wave illusion results from the relative phase of the frequency components. Adaptation to two frequency components in square-wave (sine) phase was found to reduce the illusion strength, and adaptation to triangular-wave (cosine) phase was found to increase the illusion strength. In addition, the square-wave adaptation effect spreads to nonadapted retinal areas. It is concluded that the square-wave illusion reflects a phase anisotropy in the human visual system that favors square-wave phase over other phase relations.