When taking the pictures, keeping the rules of three in mind help so that fewer modifications to the picture are needed later on. If not taken into account, rules of three will need to be applied to the editing of the picture process in order to produce a picture that is pleasing to the eye. Focal point should be centered in one of the three divisions of the picture. Use two thirds as much as possible in horizontal picture with the majority of the picture in the two thirds of the picture and less emphasis placed on the other third. Eyes of subject should go on top or bottom line of the the two thirds.
Top line indicates action.
Bottom line indicates at rest or sleep. Major is the third part of the picture that has the major emphasis of picture in it. Major emphasis should be in right two-thirds of the photo. I think this is the right one, contains some great information on photographic composition. The darkest area that you want to see the details there and take the picture with the exposure with that area in mind.