The gnomon is a control used to adjust position and orientation of coordinate frames and transform objects in the viewport.
One dimensional translation
Move the gnomon along the X, Y, or Z axis by clicking the axis and moving the cursor.
Moving the gnomon along the X axis:
2D translation
Move the gnomon along the XY, YZ, or XZ plane by clicking the plane (in gray) and moving the cursor.
Moving the gnomon on the YZ plane:
3D translation
Move the entire gnomon freely in 3D by clicking the origin and moving the cursor.
Moving the gnomon origin:
Rotation about an axis
Click the rotate axis control to rotate the gnomon about an axis.
Rotation about the Y axis:
2D rotation a
3D rotations
Cancel actions when active gnomon
Cancel by right-clicking