[[File:comparison_visual_landing_systems.svg|thumb|250px|Comparison of [[Precision approach path indicator|PAPI]], VASI, and [[Optical landing system|OLS]] meatball and datum lights {{nowrap|(not to scale)}}]]
Basic visual approach slope indicators consist of one set of lights set up some {{convert|7|m|ft}} from the start of the runway. Each light is designed so that it appears as either white or red, depending on the angle at which it is viewed. When the pilot is approaching the lights at the proper angle, meaning the pilot is on the [[glide slope]], the first set of lights appears white and the second set appears red. When both sets appear white, the aircraft is too high, and when both appear red it is too low. This used to be the most common type of visual approach slope indicator system; however, it is being phased out and replaced by [[precision approach path indicator]]s (PAPIs), which are closer together and therefore more efficient to sight and maintain.
A mnemonic to remember the colors and their meaning is:
