WindowAlert decals help prevent wild birds from accidentaly striking windows. Each decal has a special coating that reflects ultraviolet sunlight. This ultraviolet light is invisible to humans, but birds see it as a brilliant glow - much like a stoplight. To the human eye, the decals appear as frosted or etched glass. Our new Modern Square decal is popular with owners of modern homes, where steel and glass are the predominate building materials. And, as well in commercial and institutional settings where the decal is complimentary to those particular architectural styles. Window Alert decals may be used only on an exterior glass surface - free of any overlay, tinting, film, or coating. Clean glass first with water. Avoid use of chemicals such as ammonia or window cleaners. Decals are best applied when glass is warm (ideally greater than 50 degrees). If applied during winter months, clean glass with warm water prior to application. Place decals alone or in groups every few feet on the outside of the window. Position out of reach of infants and small children. Restore static cling by rinsing in lukewarm water. UV coating may fade based on exposure and local elevation. Replace decals every 9 to 12 months. There are 4 decals in a package.