A rectangular Cartesian coordinate system in three dimensions which has the positive directions on the three axes (x-, y-, z-axes) defined in such a way that if the thumb of the right hand is imagined to point in the positive direction of the z-axis and the fore finger in the positive direction of the x-axis, then the middle finger, extended at right angles to the thumb and forefinger, will point in the positive direction of the y-axis. If the coordinate system is left handed, the middle finger will point in the negative direction of the y-axis. While two oppositely oriented, rectangular Cartesian coordinate systems can be defined on a plane, neither is intrinsically right handed or left handed. However, if the plane has a positive and negative side defined, then a z-axis with positive and negative directions could be considered present implicitly and right handed and left handed coordinate systems could be defined on the plane.