Yin is the black side, and yang is the white side. The relationship between yin and yang is often described in terms of sunlight playing over a mountain and a valley. Yin (literally the 'shady place' or 'north slope') is the dark area occluded by the mountain's bulk, while yang (literally the "sunny place' or "south slope") is the brightly lit portion. As the sun moves across the sky, yin and yang gradually trade places with each other, revealing what was obscured and obscuring what was revealed.