Red String of Fate

The Red String of Fate is a romantic legend detailing that red strings exist tying two or more people together that are destined to interact in some way; be it romantically or in some other notable way, regardless of their current time, place or circumstances.

This legend was first developed after it was discovered that the ulnar artery connects the heart with the "pinky finger" (which is the same reason why in many cultures promises are made by two people crossing their pinkies). The legend goes that the thin vein running from heart to hand extends through the invisible world, to end its course in someone else’s hand and heart.

Trivia

 * Originally, in Japanese, the string was though to be linked to a male's thumb to a female's pinky finger, however, it has since become more common to show both parties attached at the pinky.
 * The Red String of Fate belief is so well known in Japan that it is invoked through linguistic gestures: holding up the thumb is used as shorthand for "girlfriend" and holding up the pinky would indicate a "boyfriend".