Take a straight line. How should one divide the length into two parts such that the ratio of the length of the whole line to the longer segment is equal to the ratio of the longer segment to the shorter one? A little algebra gives you the result that longer segment should be 0.618 times the length of the whole line and thus the ratio of the full line to the longer segment is 1.618 (=1/0.618).

The number 1.618 is known as the ‘Golden Ratio’ and folklore ascribes deep significance to it and claims a ubiquity for it that far exceeds the reality.

Mathematician Keith Devlin tries to set the record straight.