Abstract:

Traditional two-dimentional space-time diagrams are often drawn with time and space axes perpendicular to each other in
the basic coordinate system but not perpendicular in other inertial coordinate systems. And per unit length of the same coordinate axis
in different coordinate systems usually represents different physical quantity. Therefore, Huang Xianmin proposes that space-time
diagrams should be symmetric and offers a special case but never explains its general drawing method. In fact, according to geometry
and special relativity, in two coordinate systems of symmetric space-time diagrams, the space axis of one coordinate system must be
perpendicular to the time axis of the other and the time axis of one coordinate system must be perpendicular to the space axis of the
other. In this diagram, although with oblique space-time axes, per unit length of axis in two symmetric coordinate systems represents
the same physical quantity. Other useful results are also obtained.

Key words: special relativity, two-dimentional space-time diagram, oblique coordinate axes