The representation of Aikido principles by the geometrical shapes triangle, circle and square are taken from the Shinto cosmology theory of “Gogyo Gogen” where the elements of existence, Liquid, Solid and Gas are all shown to be transmutable and interrelated.
In his book “Kami no Michi“, Rev. Dr. Yukitaka Yamamoto, the 96th generation Shinto High Priest (Guji) wrote:
“The Principle of ‘Sanmi–Sangen‘ explains the mystery of life. Sanmi–Sangen means the three elements that constitute the basis of all forms of existence. These basic symbols both explain the meaning of and guide the destiny of human life. We can see Sanmi–Sangen operate at many levels.”
In incredibly simplistic terms, the Triangle can be said to represent the triangular posture predominant in Aikido, triangulation of arms to centre/hips, and triangular offline avoidance and the Square to represent solidity of stance, or the principles of “weight-underside”.