The gnomon is a control used to adjust position and orientation of coordinate frames and to transform objects in the viewport.
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Rotation about the Y axis:
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2D rotation a
3D rotations
Cancel actions when active gnomon
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Rotation about axis to align another axis
Click an arrow at the end of the rotate axis control to rotate the gnomon about an axis and align another axis.
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Example rotating about the Y axis and aligning the X axis:
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3D rotation to align axis
Click the sphere on an axis and then another point to align the axis in 3D.
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Rotating so that the Y axis aligns with a point:
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Cancel action when using the gnomon
Right-clicking when using the gnomon will cancel any moves.