When labeling coordinate axes, there are many possible ways to select the positive x
, y
, and z
directions, but it is mandatory that the resulting axes form a right-handed coordinate system. The handedness of a coordinate system may be deduced by applying the right-hand rule as follows. With the right hand held flat, align the thumb with the +i^
direction, and align the fingers with the +j^
direction. If the palm of the right hand is open towards the +k^
direction, then the coordinate system is a right-handed coordinate system with i^\times j^=k^
; if the palm and the +k^
direction are oppositely directed, then the coordinate system is left-handed.