The axes of the Polaroid filters in sunglasses should be vertical.
Work Step by Step
If the reflective surface is nonmetallic, much of the reflected light is polarized. The polarization direction is parallel to the direction of the reflecting surface. This is discussed on page 556, and diagrammed in Figure 29.33. Many reflecting surfaces are horizontal planes (roads, water, the hood of one's car, etc.), so the reflected light (glare) is polarized horizontally. Sunglasses with vertically-oriented polarization axes filter out the glare.