The Golden Mean or the Golden Ratio is a concept from both art and mathematics, that has intrigued people for centuries. At it's simplest, if you take a straight line and wish it divide it in two unequal parts according to the Golden Mean, then the ratio of the total length of the line to the larger part needs to be the same as the ratio of the larger part of the line to the smaller part. This idea of dividing a distance or an area into what are seen as pleasing proportions using the Golden Mean occurs in art, architecture and also in mathematics. This little dividing device allows you to display three measurements, all with the ratios of the Golden Mean. One example in the human body where the Golden Mean has been argued to apply is the ratio of successive phalangeal bones of the digits and the metacarpal bone, which Tim demonstrates in the video. This is a really big subject, and a quick online search will give you enough reading for a week!