Quadrupoles have four magnetic poles. In a particle accelerator, the poles push particles together if they deviate too far from the centralized beam. Quadrupoles focus in only one plane, so to squeeze an accelerator beam from both sides, these magnets are usually stacked one after the other, each rotated by 90 degrees relative to the previous one. In this way, the beam particles are pushed together in both directions as they travel through successive magnets. Image: Jerald Pinson