Most omens people still repeat are household custom: a penny, a pinch of salt, a mirror, a bird at the window. They were handed down by word of mouth, which is why two people in the same family can tell different versions of the same one.
In American folk custom, finding a penny face up is good luck, and many people read it as a hello from someone who has died. The rhyme about picking it up began as one about pins, and the tradition of lucky metal is much older.
In European folk custom, spilling salt is bad luck, and the usual remedy is to throw a pinch over your left shoulder. The belief comes from how valuable salt used to be, and the Hebrew Bible already treats salt as a sign of a lasting promise.
In European and American folk custom, a broken mirror brings seven years of bad luck. The number is often traced to the Romans, who held that life renewed itself in seven-year cycles. The belief itself is better documented than its origin.
Rainbows are read as promises, bridges and messengers. In the Hebrew Bible a rainbow is the sign of a covenant after the flood, in Norse myth it is a bridge between worlds, and in Greek myth it is a goddess carrying messages.
In British, Irish and American folk custom, a bird at a house or inside it is often read as a sign of death. The belief is widespread, and it is easy to see why it grew: a bird is a creature of the outside suddenly inside.
No single source explains it. The usual strands are thirteen at the Last Supper, Friday as the day of the crucifixion, and a later fear that merged them. The widely told story about the Knights Templar is popular and has weak support.
Nobody knows for sure. The theories include old beliefs about spirits in trees, a link to the cross, and a Victorian children's game. The folklorist Steve Roud favors the game.
