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. To avoid confusion, Latin terminology is employed: dexter and sinister for right and left. Proper right and proper left are terms mainly used to describe artistic images, and overcome the potential confusion that a figure's "own" right or "proper right" hand is on the left hand as the viewer sees it from the front.
Forward and backward may be defined by referring to an object's or person's motion. Forward is defined as the direction in which the object is moving. Backward is then defined as the opposite direction to forward. Alternatively, 'forward' may be the direction pointed by the observer's nose, defining 'backward' as the direction from the nose to the sagittal border in the observer's skull. With respect to a ship 'forward' would indicate the relative position of any object lying in the direction the ship is pointing. For symmetrical objects, it is also necessary to define forward and backward in terms of expected direction. Many mass transit trains are built symmetrically with paired control booths, and definitions of forward, backward, left, and right are temporary.
Given significant distance from the magnetic poles, one can figure which hand is which using a magnetic compass and the sun. Facing the sun, before noon, the north pointer of the compass points to the "left" hand. After noon, it points to the "right".
A right-hand rule is one common way to relate three principal directions. For many years a fundamental question in physics was whether a left-hand rule would be equivalent. Many natural structures, including human bodies, follow a certain "handedness", but it was widely assumed that nature did not distinguish the two possibilities. This changed with the discovery of parity violations in particle physics. If a sample of cobalt-60 atoms is magnetized so that they spin counterclockwise around some axis, the beta radiation resulting from their nuclear decay will be preferentially directed opposite that axis. Since counter-clockwise may be defined in terms of up, forward, and right, this experiment unambiguously differentiates left from right using only natural elements: if they were reversed, or the atoms spun clockwise, the radiation would follow the spin axis instead of being opposite to it.
Bow, stern, port, starboard, fore and aft are nautical terms that convey an impersonal relative direction in the context of the moving frame of persons aboard a ship. The need for impersonal terms is most clearly seen in a rowing shell where the majority of the crew face aft ("backwards"), hence the oars to their right are actually on the port side of the boat. Rowers eschew the terms left, right, port and starboard in favor of stroke-side and bow-side. The usage derives from the tradition of having the stroke (the rower closest to the stern of the boat) oar on the port side of the boat.
Most human cultures use relative directions for reference, but there are exceptions. Australian Aboriginal peoples like the Guugu Yimithirr, Kaiadilt and Thaayorre have no words denoting the egocentric directions in their language; instead, they exclusively refer to cardinal directions, even when describing small-scale spaces. For instance, if they wanted someone to move over on the car seat to make room, they might say "move a bit to the east". To tell someone where exactly they left something in their house, they might say, "I left it on the southern edge of the western table." Or they might warn a person to "look out for that big ant just north of your foot". Other peoples "from Polynesia to Mexico and from Namibia to Bali" similarly have predominantly "geographic languages". American Sign Language makes heavy use of geographical direction through absolute orientation.[clarification needed]