Final answer:
The establishment of proportions in a design to create symmetry is known as Balance, which provides a sense of stability and formality in art compositions, as seen in works such as Perugino's painting and natural examples like the Moon Jellyfish.
Step-by-step explanation:
Establishing equal or appropriate proportions to create symmetry in a design is called Balance. This principle of art refers to the distribution of elements to create a sense of stability and calm. Symmetrical balance is achieved when parts of a design are mirror images of each other across an axis, providing visual stability and formality. An example of this would be the bilateral symmetry seen in Perugino's painting, Christ Giving the Keys of the Kingdom to St. Peter, where the symmetry contributes to a sense of balance as well as a serene and structured atmosphere.
The concept of symmetry is also evident in the natural world, such as the Moon Jellyfish, which displays an almost perfect symmetrical balance in its design. The practice of creating balance through symmetry is a fundamental aspect of art composition that guides the viewer's eye and imparts a sense of harmonious proportion.