In special relativity, light-cone coordinates is a special coordinate system where two of the coordinates, x+ and x− are null coordinates and all the other coordinates are spatial. Call them x ⊥ {displaystyle x_{perp }} . In special relativity, light-cone coordinates is a special coordinate system where two of the coordinates, x+ and x− are null coordinates and all the other coordinates are spatial. Call them x ⊥ {displaystyle x_{perp }} .