The two third rule means you would either have two thirds of your picture as sky or two thirds of your picture as landscape i.e. water, land mass etc. It is usually wise not to have the horizon line splitting the picture in the middle. I have done pictures where the horizon line was more or less in the middle but you this means you have to have a lot beneath it and a lot above it.
The picture on the left shows a badly composed picture of a famous Scottish Castle. The horizon line is more or less in the middle splitting the picture in two at the same time positioned right in the middle completely dominating the picture all the other lovely scenery is left wanting.
This second picture is very well composed the position of the horizon line gives us a larger amount of sky that can be enhanced using artistic license with just enough water below the horizon line to allow reflections etc. The Castle is to one side giving us a better view into the distance, all together a better composition and one that is on my list to paint for the future. When I do, the tutorial will be available for members of my pastel pencil art courses. Click Here to Learn How To Join.