The fist lesson of composition for any photography course is the Rule of Thirds. Everyone who is into photography should really master this technique because it balances everything up creating more solid pictures. Although breaking this rule sometimes makes some photos highly captivating.
It is natural to me that when I take pictures, I always compose it like this so, I don’t really use the grid in my viewfinder but for the sake of exploring the Nikon d5000, I enabled the grid view. It doesn’t have the usual 3×3 squares, instead it has the 4ths but the main points of focus were in the open spaces. So, I just figured the open spaces are the imaginary lines for the Rule of Thirds, the main focus points are of the intersections and this visible lines are for something else. Maybe for better leveling when doing landscape, etc.