You may have heard photographers talk about the rule of thirds. If you haven’t then what this means is then when composing a photograph, it will always look more balanced and effective if the photograph is snapped such that the subject matter is arranged in an aspect of thirds or 2/3 to 1/3.
To look at an example from the art world, if you take Da Vinci’s last supper you will see that it can be divided up into thirds. This is a kind of Golden Ratio, artists and designers have been using these proportions to create aesthetically pleasing works and this rule also works in Web Design.
Websites usually benefit from being arranged into columns or sections. These columns generally “look right” when arranged into thirds. So if you are going to have 2 columns, these will look best if arranged with one column 1/3 wide, the other 2/3 wide. If you are going to have 3 columns then that will be fine too. 4 columns won’t work as well.
If you take a look at your favourite websites and imagine a kind of grid over them, you will find that almost all of them obey the rule of thirds.