It is hard to classify Metro UI and Material Design as flat design or neumorphism, mainly because both design languages make use of depths. Metro UI employed a heavy dose of parallax effect, which implied a sense of depth; Although parallax effects are also seen in Material Design, drop shadows are more often used to hint on the depths of objects in Google's design language. Drop shadow, since at least Windows XP era, has been around in Microsoft Windows operating systems. Since both of the design languages shared the metaphor of z-axis depth, usage of such property may not warrant a clear separation between flat design and neumorphism.