Threefold Inversion Axis (-3)

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The symbol above shows a threefold inversion axis, i.e. a threefold axis perpendicular to the screen with a centre of inversion on it. A fractional value next to the symbol indicates the height of the inversion centre above the XY plane. As with the threefold axis, a positive rotation is anticlockwise. A threefold rotation about the c-axis, i.e. about the line 0,0,z, followed by an inversion at the origin will have the corresponding symmetry operator y,-x+y,-z. This axis has the written symbol "-3" (where the minus sign is usually written above the digit).

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