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bipyramid In crystallography, a pyramid defines a crystal face which cuts the vertical ‘c’ (or ‘z’) axis and one or two of the horizontal ‘a’ (or ‘x’) and ‘b’ (or ‘y’) axes. In general, the prefix ‘bi-’ refers to the repetition of faces about a plane of symmetry; ‘di-’ usually suggests the presence of an axis of symmetry about which the pyramid may be rotated.