event
Fig. 3-1a. Draw the 



x


{\displaystyle x}

 and 



t


{\displaystyle t}

 axes of frame S. The 



x


{\displaystyle x}

 axis is horizontal and the 



t


{\displaystyle t}

 (actually 



c
t


{\displaystyle ct}

) axis is vertical, which is the opposite of the usual convention in kinematics. The 



c
t


{\displaystyle ct}

 axis is scaled by a factor of 



c


{\displaystyle c}

 so that both axes have common units of length. In the diagram shown, the gridlines are spaced one unit distance apart. The 45° diagonal lines represent the worldlines of two photons passing through the origin at time 



t
=
0.


{\displaystyle t=0.}

 The slope of these worldlines is 1 because the photons advance one unit in space per unit of time. Two events, 




A



{\displaystyle {\text{A}}}

 and 




B

,


{\displaystyle {\text{B}},}

 have been plotted on this graph so that their coordinates may be compared in the S and S' frames.
