X is the sign of our times; simultaneously ubiquitous yet elusive. From uncertain origins as a mark of affection, x has emerged as a universal symbol for those hard to define qualities, as well as youthful independence, attraction, danger, and excitement. From a potent (X rating), to an unknown energy (X-ray), to a secret code (Station X at Bletchley Park), to the TV programme (X Factor), or even the religious festival (Xmas); x has become the defining letter of the millennium.