Golden section (phi)
(top) cut in a rod, dividing it into Major and minor, such as m:M = M:(M+m), or: the small part relates to the big part as the big one to the whole

phi = M/m = 1.6180339..., or (1+V5)/2 = t(n)/t(n-1) in the Fibonacci series, while n approaches infinity
M/m = m/M+1
phi^2=phi+1

(top right) In the pentangle, all connections are divided like phi. Phi in this square, triangle, and circle is remarkable. In many natural structures, like the human body, the Golden Section can be traced.

(below) In art and architecture, there is a tradition in applying the Golden Section.