前、后、左、右、上、下都是表示相对方向的语汇。它们随某个人的面对方向变化而变化。人们互相之间容易混淆这些方向，比如说"左" 这个方向对于一个人和另一个人可能是不一样的。显然它们与「东西南北」这类固定的方向区别很大，相对方向是按照人的视角决定的。

虽然是相对方位，但在不同的文化里，「上、下、左、右」都有不同程度的其它含义（前与后的含义比较明确：前代表眼睛能看到的，所谓「眼前」；后则是其相反方向，「背后」）。在汉语里，有「上北、下南、左西、右东」的习惯说法。

The most common relative directions are left, right, forward(s), backward(s), 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. There are definite relationships between the relative directions, however. Left and right, forward and backward, and up and down are three pairs of complementary directions, each pair orthogonal to both of the others. Relative directions are also known as egocentric coordinates.