A mathematical proportion that is considered to be particularly pleasing to the eye, where “the smaller is to the larger as the larger is to the sum of the two”. The quick, off-the-cuff calculation is 5:8 or 1:1.6
This principle echoes spirals and most natural formations, and much of successful and aesthetically pleasing architecture and design draws upon the golden section, even if for reasons of practicality or restriction the exact proportions have been manipulated or adjusted.
This diagram shows the build up of the golden section rule, into useable form.