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