Our surroundings make sense to us because of how light hits their images causing consequent shadows. We can reduce these lights and shadows to black and white and it will make a visual thought, show us a visual structure where darks connect and light carries a path within and around the darks . And this will give us the Notan of the scene.
Photo to notanNotan, pronounced "no tan", is a Japanese concept meaning "light dark". It is related to yin yang where yang is light and yin is dark. One way to give solid structure to a painting is to do just that--designate all lower value notes to black, all higher notes the white. I used the word "notes" because if we can use music as an analogy and we can easily understand the vocabulary of value.
On a piano there is middle C which is the center of the notes available on the piano keyboard. Notes to the left of middle C are lower in tone, notes to the right are higher. Light and dark notes can be similar, but because the value range in nature is so vast, we get a better grasp to limit value notes to 10 where 1 is the lightest or highest and 10 is the darkest or lowest, 5.5 would be equal to middle C.
Using this system of thinking, we can find our true pattern of lights and shadows if we create a notan, placing value notes 1-5.5 in the white areas and notes 5.5-10 in the blacks.
This is most easily done in a tiny drawing no bigger than 1" x 2". When creating the drawing, we can decide whether we want the light pattern as we perceive it to be or if we want it
or lower key .
From here, we can create our painting, keeping our color values one-ish to five-ish where the notan indicates light and five-ish to ten-ish where it indicates dark, referring back to our source, whether plein air or photo, for the colors we want to use.
Notan is only one way of guaranteeing a successful value structure, but it's also a surefire way of getting a strong design within which to compose a painting. Go here then here to see I use notan.