Most runways with a published IFR approach are equipped with lights that help pilots determine if they're on the proper descent path during the final stages of approach and landing. The most common system is the VASI, or Visual Approach Slope Indicator.

VASIs come in several varieties, but a typical system includes two sets of light bars placed on the left side of the runway, one nearer the landing threshold than the other. Lenses split the light into red and white beams.

Three combinations of lights exist with this arrangement. If you're approaching the runway on the proper glide path, usually a three-degree slope, you'll see a red light above a white light.

Left, "Red over red, you're dead," approach is low. Center, "Red over white, you're all right," approach is perfect. Right, "White over white, you'll fly all night," approach is high.

Intuitively, of course, one understands that red-red is "danger," or low, and then the others take care of themselves.