spacetime diagram
A spacetime diagram is a simple graph that turns relativity into a picture you can read with your eyes. Time runs up the page and space runs across it, so a single dot stands for an event — something happening at one place at one moment. By convention the time axis is scaled in light-distance (using c·t), which makes the rules come out clean and symmetric.
On this diagram a flash of light always travels along a 45° line, because light covers one unit of distance in one unit of time. Anything slower than light — a person, a planet, a spaceship — traces a steeper, more vertical path, since it covers less distance per tick of the clock. This single fact, that light goes at 45°, is the seed from which simultaneity, time dilation, and length contraction all visibly grow.
The real magic appears when you draw a second observer moving past the first. Their time and space axes tilt toward the 45° light line like the closing blades of a pair of scissors, yet light keeps its 45° slope for both of them. Reading off where the tilted axes cross the worldlines shows, at a glance, why two people can honestly disagree about which events happen 'at the same time'.
Plot c·t upward and x across, and every light ray runs at exactly 45°.
On a moving observer's diagram the axes are tilted (not perpendicular), and units along them are rescaled — so distances on the page are not the spacetime interval. Always read events off the worldlines, not off the paper's ruler.