Asymmetry is the absence of, or a violation of, symmetry. Symmetry is an important property of both physical and abstract systems and it may be displayed in precise terms or in more aesthetic terms. Asymmetry can be observed in various fields, including biology, design, and art. In biology, asymmetry is an important and widespread trait, having evolved numerous times in many organisms and at many levels of organization. Benefits of asymmetry sometimes have to do with improved spatial arrangements, such as the left human lung being smaller and having one fewer lobes than the right lung to make room for the asymmetrical heart. In design, asymmetrical balance results from unequal visual weight on each side of the composition, and it offers more visual variety, although it can be more difficult to achieve because the relationships between elements are more complex. Asymmetrical balance can be used to create visual tension and evoke feelings of modernism, movement, energy, and vitality. In art, asymmetrical balance can be used to create a sense of balance by playing with the size and color of different elements. Asymmetry can also refer to something that is not identical on both sides of a central line, lacking symmetry.