The Rules of Thirds is a universal concept that artists and designers use by creating a grid of nine boxes in order to draw the user’s eye to specific areas on the design. The idea is straightforward; you place a simple grid overlay (divided equally into thirds, both horizontally and vertically) on the space to be used for the design. The most important part of the grid is where the lines cross. These are focal points or sweet spots.
You may have already heard this term before when talking about photography and compositions, but it definitely applies to web design as well.
As users and designers, we generally scan an image or layout visually in an F-shape. By starting with most of our focus given to the top-left sweet spot, we look down to the bottom-left sweet spot before going up to the top-right one, and then, and with the least attention, we scan the bottom-right sweet spot.
We can use this information to make sure we place our calls to action and most important info in these sweet spots.