Spacetime diagrams are powerful, but they can also be confusing. In this and the next five demonstrations, we isolate various features of spacetime diagrams to help build your understanding step-by-step. This demonstration illustrates the spatial slices of a moving frame. Use the slider to choose the velocity of the moving frame. Then observe how the $x^\prime$-axis (labelled $t^\prime=0$ in the diagram because it represents all points in space at the moment $t^\prime=0$) rotates. Notice that the angle of rotation is always less than 45º, reflecting that $v$ is always less than $c$. Next, click the button to show spatial slices at other moments of time (i.e., for other values of $t^\prime$) in the moving frame. Finally, use the slider to change the speed of the moving frame and notice how the spatial slices rotate. (Convince yourself of the following: The fact that the spatial slices in the moving frame are not horizontal is the graphical version of the relativity of simultaneity.)