<P> When pointing the thumb away from the origin along an axis towards positive, the curvature of the fingers indicates a positive rotation along that axis . </P> <P> Regardless of the rule used to orient the axes, rotating the coordinate system will preserve the orientation . Switching any two axes will reverse the orientation, but switching both will leave the orientation unchanged . </P> <P> Once the x - and y - axes are specified, they determine the line along which the z - axis should lie, but there are two possible directions on this line . The two possible coordinate systems which result are called' right - handed' and' left - handed' . The standard orientation, where the xy - plane is horizontal and the z - axis points up (and the x - and the y - axis form a positively oriented two - dimensional coordinate system in the xy - plane if observed from above the xy - plane) is called right - handed or positive . </P> <P> The name derives from the right - hand rule . If the index finger of the right hand is pointed forward, the middle finger bent inward at a right angle to it, and the thumb placed at a right angle to both, the three fingers indicate the relative directions of the x -, y -, and z - axes in a right - handed system . The thumb indicates the x-axis, the index finger the y - axis and the middle finger the z - axis . Conversely, if the same is done with the left hand, a left - handed system results . </P>

Why do we use x and y axis