Pen tilt
Pen tilt refers to the angle of the writing instrument during the handwriting process, which can vary over time. In a coordinate system which is determined by the writing surface plane 
and the vertical pen-tip movement along the 
axis, all three two-dimensional planes can be discerned, and the angular signals can be delivered by a digitizer:
The rotation of a pen along its longitudinal axis is seldom taken into consideration, as some writers have a habit of continuously rotating the pen as they write. This phenomenon became apparent via electronic pens tethered to the digitizer, whose cords became immensely curled.