Body relative direction

Body relative directions (also known as egocentric coordinates) are geometrical orientations relative to a body such as a human person's body. The most common ones are: left and right; forward(s) and backward(s); up and down. They form three pairs of orthogonal axes. Since definitions of left and right based on the geometry of the natural environment are unwieldy, in practice, the meaning of relative direction words is conveyed through tradition, acculturation, education, and direct reference. One common definition of up and down uses gravity and the planet Earth as a frame of reference. Since there is a very noticeable force of gravity acting between the Earth and any other nearby object, down is defined as that direction which an object moves in reference to the Earth when the object is allowed to fall freely. Up is then defined as the opposite direction of down. Another common definition uses a human body, standing upright, as a frame of reference. In that case, up is defined as the direction from feet to head, perpendicular to the surface of the Earth. In most cases, up is a directionally oriented position generally opposite to that of the pull of gravity. In situations where a common frame of reference is needed, it is most common to use an egocentric view. A simple example is road signage. Another example is stage blocking, where "stage left" "stage right" are, by convention, defined from the point of view of actors facing the audience. "Upstage" and "downstage" do not follow gravity but by convention mean away from and towards the audience. An example of a non-egocentric view is page layout, where the relative terms "upper half" "left margin," etc. are defined in terms of the observer but employed in reverse for a type compositor, returning to an egocentric view. In medicine and science, where precise definitions are crucial, relative directions (left and right) are the sides of the organism, not those of the observer. The same is true in heraldry, where left and right in a coat of arms is treated as if the shield were being held by the armiger.