Cardea or Carda was the ancient Roman goddess of the hinge (Latin cardo, cardinis), Roman doors being hung on pivot hinges. The Augustan poet Ovid conflates her with another archaic goddess named Carna, whose festival was celebrated on the first day of June and for whom he gives the alternative name Cranê or Cranea, a nymph. Ovid's conflation of the goddesses is likely to have been his poetic invention, but it has also been conjectured that Carna was a contracted form of Cardina, and at minimum Ovid was observing that their traditions were congruent.

In the Christian polemic of the Church Fathers, Cardea is associated with two otherwise unknown deities who preside over doorways: Forculus, from fores, "door", plural in form because double doors were common on public buildings and elite homes (domūs); and Limentinus, from limen, liminis, "threshold" (compare English "liminal"). St. Augustine mocks the apparent triviality of these "little gods" in one of his "attacks against the multitude of gods", noting that while one doorkeeper is adequate for a human household, the Roman gods require three: "evidently Forculus can't watch the hinge and the threshold at the same time." Modern scholarship has pointed out that this particular set of divinities belongs to rituals of marking out sacred space and fixing boundaries, religious developments hypothesized to have occurred during the transition from pastoralism to an agrarian society. Among Roman deities of this type, Terminus was the most significant.

Stefan Weinstock conjectured that these three doorway deities had a place in cosmology as the Ianitores terrestres, "doorkeepers of the earth", guarding the passage to the earthly sphere. In the schema presented by Martianus Capella, the Ianitores terrestres are placed in region 16 among deities of the lowest ranks, while Janus, the divine doorkeeper par excellence, is placed in region 1. This arrangement may represent the ianuae coeli, the two doors of the heavens identified with the solstices. Isidore of Seville says that there are two ianuae coeli, one rising (that is, in the East) and one setting (the West): "The sun advances from the one gate, by the other he recedes."

Isidore's definition is followed immediately by an explanation of the cardines (plural of cardo), the north-south pivots of the axis on which the sphere of the world rotates. These are analogous to the top-and-bottom pivot hinges of a Roman door.

In addition to the meaning of "door hinge", the cardo was also a fundamental concept in Roman surveying and city planning. The cardo was the main north-south street of a town, the surveying of which was attended by augural procedures that aligned terrestrial and celestial space. The cardo was also a principle in the layout of the Roman army's marching camp, the gates of which were aligned with the cardinal ( a word derived from Latin cardo/cardinis) points to the extent that the terrain permitted.

Macrobius (5th century) says that the name Carna was derived from caro, carnis, "flesh, meat, food" (compare English "carnal" and "carnivore"), and that she was the guardian of the heart and the vital parts of the human body. The power to avert vampiric striges, which Ovid attributes to the conflated Cardea-Carna, probably belonged to Carna, while the charms fixed on doorposts are rightly Cardea's.

Carna's feast day was marked as nefastus on the calendar; that is, it was a public holiday when no assembly or court could convene. Mashed beans and lard – a dish perhaps to be compared to refried beans or hoppinjohn – were offered to her as res divinae, and thus the day was known as the Kalendae fabariae, the Bean-Kalends, since at this time the bean harvest matured. Beans had many magico-religious properties in ancient Greece and Rome in addition to their importance as a food crop.

William Warde Fowler took Carna to be an archaic goddess whose cult had not been revivified by religious innovation or reform and thus had lapsed into obscurity by the end of the Republic. Auguste Bouché-Leclercq considered Carna a goddess of health. Her elusive nature is indicated by the wildly divergent scholarly conjectures she has prompted: "she was considered a chthonic divinity by Wissowa, a lunar goddess by Pettazzoni, a bean-goddess by Latte, and a patroness of digestion by Dumézil".