Oracle decks are unusual among the subjects in this atlas because the category is defined negatively. A tarot pack has a fixed architecture: four suits, ten numbered cards and four courts in each, and twenty-two trumps, and any deck departing from that is departing from something. An oracle deck has no such constraint. It may have any number of cards, any subject matter and any internal arrangement, and its designer decides all of it.
The immediate ancestor is the Lenormand, which took shape in the early nineteenth century out of a German parlour game and attached the name of a celebrated French cartomancer after her death. Its thirty-six cards carry plain objects, a ring, a ship, a coffin, a bouquet, and are read in combination rather than singly, which is a genuinely different technique from tarot reading.
The twentieth century multiplied the form. Decks appeared on angels, animals, plants, goddesses, crystals, and the imagery of particular artists, and cheap colour printing then made it feasible for an individual to produce one. The result today is an open genre in which most decks are self-published and come with the designer's own booklet supplying the meanings.
That openness cuts both ways. It leaves the tradition free of the corrections a shared structure imposes, and it makes an oracle deck an unusually direct expression of one person's idea of what matters, which is part of why readers choose them.