The golden ratio is a mathematical equation determined by ancient Greek scholars to explain natural ratios. The ratio uses the irrational number 1.61803 to 1. It's still used in Western culture to create pleasing art ratios.
In common practice, the ratio is roughly 2 to 3. It applies both vertically and horizontally.
This ratio also happens to be the same approximate ratio as most film formats. Consequently, as most images are made, they conform to the golden ratio. This also explains why 8 by 10 prints are not as faithful to the negatives as 8 by 12 prints are.
The golden ratio is also called the divine proportion, golden mean, golden number, golden proportion and golden section.
See additional information Use the Golden Ratio on the PhotoJournalism blog.