Definition:  chiaroscuro: [It., f. chiaro clear, bright + oscuro dark, OBSCURE a.].  2 The treatment or disposition of the light and shade, or brighter and darker masses, in a picture; an effect or contrast of light and shade in a picture or in nature. L17. [The Oxford Interactive Encyclopedia. (c) 1997 TLC Properties Inc.]

      Since at least the Renaissance (but maybe earlier and we just do not have any physical evidence) it has been customary for painters to do tonal studies, black and white sketches while planning the composition of future major works. Chiaroscuro is arguably the most powerful technique at a painter's disposal for evoking mood with-in the composition. [ see here for others ]. The mood so created by the play of light and shade within the composition, has the added advantage of being independent of any culturally biased colour symbolisms.

By the mid nineteenth century different tonal schema had come to-be associated with particular genres (classes of subject matter) of paintings, or works by a given painter during phases of the life, such as Goya's black paintings, or Rembrant's golden landscape. Then in the mid twentieth century a musical metaphor of scales was employed to distil the various templates like "camp fire scene", "bright sunny day" into approximately eight 'tonal keys'.  The terminology 'tonal key' was unsatisfactory for a number of educational and other reason, so [ as explained elsewhere ] I settled on the designation "value regime".

But before examining 'value regimes' one needs to better understanding of what exactly is meant by 'value'. Traditional discussion of this topic was clouded first by a dichotomy of 'highlights & shadows' or 'light & shade'. 'Tonal steps' then became the common description, but 'tone' was exclusively associated with achromatic (non-colour) compositions, with the dichotomy persisting in the colour realm as 'pastels, tints & shades'.  Over the twentieth century "Value" then slow displaced the previous clutter of terms to provide a uniform descriptor for gradations of shading in both achromatic and chromatic worlds. 
      While not obligatory to any understanding of 'value regimes' the eleven step gradation of value from absolute black at zero to blinding pure white light at ten, as applied by Albert H. Munsell is the easiest to use. {A note of caution, is that some graphic arts text, reverse this ordering so that one is the lightest value, as it has negligible cross-hatching to nine being a lot of cross-hatch and thus the darkest value}. Limiting our selves to nine (from pigment black at one to pigment white at nine) value step of the every day world, these can be further clustered as three bands of three steps each {see figure}The "Low" band is; black (1), 2 & 3, then 4, 5 & 6 make the "Intermediate" band, leaving the "High" band as 7, 8 and pure white pigment as 9.

All 'value regimes' as described by a two part name. The first part of the name tells you which band the greatest part of the composition is in . If most of the composition is found to have values in the 'High' band, then it is proper to speak of a work as using a 'High value regime' or to be in a 'High Regime'.
      The second part of the name indicates the importance of contrast to the schema. It is a common pitfall to regard any and every little highlight or line of shadow within the frame as critical in determining an image's value regime. Rather what must be focused on is areas of value that are substantial enough to impact on the overall value schema. There can be no hard and fast stipulation about how large an area has to be before it becomes significant to the value regime, as the style (hard-edge versus softer touch), scale and complexity of chiaroscuro within the work all subtlety alter perceptions of importance. However total areas for a value step around 10% ~ 20% ( of available  surface area) is a reasonable rule of thumb

So to ascertain the 'contrast reach' in question, examine the range in value steps between, the brightest 'significant' area compared to the darkest 'significant' area. A contrast reach of greater than six steps is designated as a "Major", thus 'Low Major' , 'Intermediate Major', and finally a 'High Major' are the only three Major value regimes. 

Compositions with maximum value reaches less than six steps make up the remaining five "Minor" value regimes. High Minor, Low Minor are fairly easy to understand, High Minor is a composition that is mainly in the High band with the darkest value being no lower than the Intermediate. Then Low Minor is just the reverse of the High Minor situation.. The three remaining Intermediate Minors are somewhat more complex.
     To recap an Intermediate Minor composition will be one where over-all impression of the image belongs to the Intermediate value band, as well as having a Minor contrast, that is to say a compressed value range or reach between the brightest and darkest significant spots in the image field. If the value distribution is on the lighter side it is a Intermediate-High Minor (aka High-Intermediate Minor). Whereas if the if the bias is towards the shadows it is a Intermediate-Low Minor (aka Low-Intermediate Minor). When the value range is clustered fairly evenly then it is an Intermediate Minor (aka Intermediate- Intermediate Minor). {see figure }.

Returning to the emotive effect of 'value regime' choice, the Major & Minor Regimes parallel the common association in music of Major & Minor Keys / Scales. Additionally Major Regimes tend to be dramatic, compared to the some-what subdued Minor Regimes. Low band Regimes are more depressed, tied or foreboding than other band regimes. High band Regimes often have a airier, lighter sometimes frivolous quality. But in the final analysis the emotive potential of value regimes is only one of many visual elements & principals that are artist marshal's to their creative ends.

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