"PHI WINDOW": Golden Ratio
Golden Ratio is a proportion that ir widely used in European arts and architecture. It is based on the Phi number that is equal to 1.61803.
This is the facade of Pažaislis church (near Kaunas). Here you can see how the Phi number is used in organizing horizontal elements. The most important numbers here are 38.2% and 61.8%, because their ratio is approximately 1.61803. The same is with 38.2% and 23.6% (76.4% and 61.8%).
The "Phi window" (below) also has ratio 1.61803. If a form (in this case it is the facade of Pažaislis) fits into it, it means it is constructed according to the same proportion.