Here's why:

* Transverse waves have oscillations that are perpendicular to the direction the wave travels. Think of a rope tied to a wall, where you shake the rope up and down. The wave travels horizontally, but the rope itself moves vertically.

* Longitudinal waves have oscillations that are parallel to the direction the wave travels. Imagine pushing and pulling a spring back and forth. The wave travels along the spring, and the coils of the spring move back and forth in the same direction.

S waves, or secondary waves, are a type of seismic wave that travel through the Earth's interior. They cause the ground to move from side to side or up and down, perpendicular to the direction the wave travels. This makes them transverse waves.