Shades of Gray
Gray and the various shades of it can be seen as the life blood of monochrome photography. While whites and blacks form the body of the work, gray is the element that bonds the other 2 together. This is the reason why black and white photography is also called grayscale, as it is the bulk of the monochrome tonal range, with pure black and white being the ends of said range.
When you look at a scene with the intention of taking a photo, chances are that a lot of the elements within the scene will be medium toned, with very few being at the ends of the aforementioned tonal range, Now, when you take the photo in black and white, these medium tones are what will show up as gray, with the darker tones being black or white. What you will notice here is that even if there was a multitude of colors in the scene, the end result will simply be various shades of gray.
Gray itself is a combination of black and white, as colors. In fact, if you look at the technical aspect of it, black and white are simply the darkest and lightest shades of gray, respectively. The rules of dynamic monochrome dictate that you show these two shades in the majority of the composition, with the middle shades occupying minimal frame space. This can bring about a very dramatic and active photographic result, but it tends to take away from the character off the image, to an extent.
Gray is used to give the photos a bit of a softer feel, while at the same time adding a touch of mystery and character to them. This can be done by shooting at the standard exposure, with care taken as to not include many sharply black or white tones in the image. Shades of gray can be found in abundance in nature itself, as well as almost any scene that you come across.
If you photograph moving or falling water, in the evening, with a longer shutter, chances are that it will show up as gray, with minimal texture. Similarly, if you shoot mist and fog, it will show up as gray as well. Both these phenomena are due to the lack of texture and detail in both photos, which created a blurry effect in the photo.