Value (in colour theory) refers to the lightness or darkness of a colour or tone, independent of its hue or saturation. In the Munsell system, value runs from 0 (absolute black) to 10 (pure white), with mid-grey at 5. Value is the most fundamental element of pictorial structure: a painting with strong value contrast reads clearly; one with compressed values (all mid-tones) becomes flat and hard to read. The ability to manage value relationships — to understand how colours that are different in hue may be equivalent in value, and thus will 'cancel' each other in a composition — is central to classical painting training. Rembrandt's mastery of value (his concentrated lights surrounded by deep shadows) is a primary reason his paintings read so powerfully across time and reproduction. Ansel Adams's Zone System in photography is essentially a systematic approach to value management.