Also called the absolute ratio, it’s found everywhere in nature. The lines created by the golden ratio are used in image composition to create a more aesthetic image to the human eye.
The official golden ratio number is: 1.618… Visually, this is what it looks like:
Some may think that the rule of thirds actually derives from this ratio but that’s not quite the case. The rule of thirds divides the image into 9 EQUAL parts as seen here:
As for the Golden ratio, the image is devided in a Phi grid based on the intersecting lines of the spiral as seen here:
Many great companies have used the golden ratio in their logo designs as well, like this one from Apple.
Here’s an excellent video explaining the difference between the Golden Ratio and the Rule of Thirds.