Countershading is an extremely common adaptation present in many species and utilized by both predator and prey to avoid detection. In simplest terms it refers to anything that is darker colored on the top and lighter underneath. It's believed that this patterning is a form of camouflage that hides an animal by imitating the background when viewed from above and below. A flying bird (a sparrow, for example) only has its white belly visible from below which helps it blend in with the sky. When viewed from above, the darker brown plumage resembles the ground, again making it less conspicuous. It is most widely used by ocean life presumably because the ocean offers few other ways to avoid being seen. In the modern world, countershading has been increasingly used in military technology especially, (as in nature) on aircraft and submarines.