WindowAlert decals help prevent wild birds from accidentally 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. Place decals on the outside of the window. Concentrate decals at center then work out placing decals at a minimum on every 2 feet of glass. For optimal protection scientific studies suggest placing decals 4 inches horizontal by 2 inches vertical from one another. UV coating will fade based on exposure and local elevation. For best results replace decals every 4 months or test with the WindowAlert UV Flashlight. CONTENTS: 4 Decals per Envelope. Each decal is approximately 4 inches across. TYPE: Static cling. Made in the USA. (If a single bird repeatedly attacks your windows please use our Stop Bird Attack product.)