The most common relative directions are: left, right, forward, backward, up, and down.

No absolute direction corresponds to any of the relative directions. This is a consequence of the translational invariance of the laws of physics: nature, loosely speaking, behaves the same no matter what direction one moves. As demonstrated by the Michelson-Morley null result, there is no absolute inertial frame of reference.

This is not to say that the relative directions cannot be defined in a non-anthropocentric way. While the location indicated by utterances involving relative direction words is dependent on perspective of the viewer, speaker, or listener, left and right (and the other relative directions) are not merely conventional.

Surprisingly, nature distinguishes between left and right, or more precisely, left-handed and right-handed helicity. Left and right can be defined precisely by observing parity violation in decays mediated by weak interactions. We can communicate the concepts of left and right to other intelligent beings who have never seen or met us, so long as their universe is governed by the same laws of nature as ours.

These definitions of left and right 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 object, down is defined as that direction which an object moves in reference to the Earth when the object is left 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, peripendicular to the surface of the Earth.

Forward and backward may be defined by referring to an object or person's motion. Forward is defined as the direction in which the object is moving. Backwards is then defined as the opposite of forward.

The terms left and right are derived from the fact that some 90% of the population use the hand of the same side of their body for writing and other manual tasks. Through cognate of the word right in the meaning correct or good, the side with the hand most used was defined as the right side. The word left comes from the Old English lyft, meaning weak. This dichotomy can also be seen in the words dextrous and sinister, from the Latin words for right and left. In order to figure out which hand is which you will need a compass and the sun. Face the sun before solar noon and check the compass. The hand on the south side is right. Left is then defined as the opposite of right.

In the vast majority of the population, the heart is slightly to the left of the body's centre line and the liver is to the right. Less visibly, the two hemispheres of the brain differ in function.

In nautical usage, the halves of a ship are designated port (left) and starboard (right), relative to a person looking forward (towards the bow of the ship).

Example

In this diagram, showing a road with right-hand traffic, the red car is to the left of the blue car. The blue car is, therefore, on the right-hand side. Should the blue car move backward, it would reach the position of the yellow car, causing an accident. For the red car to be where the green car is, it would have to move forward.

The lane containing the green and red cars is the left lane, the lane with the yellow and blue cars is the right lane.

Definition of left and right by observing parity violation in decays mediated by weak interactions must assume the definition of the magnetic field, which must assume the direction of the field lines by the right hand screw rule. Therefore, left and right must be geometrically defined first, and then re-definition of left and right by the parity violation can be verified.

Orientation

Links und rechts | ימין ושמאל (כיוונים במרחב) | Links en rechts (richting) | 相对方位