The three deities who are most commonly referred to as the "Capitoline Triad" are a group that supplanted the original Archaic Triad. This group, mirroring the Etruscan divine triad, consisted of Jupiter, the king of the gods; Juno (in her aspect as Iuno Regina, "Queen Juno"), his wife and sister; and Jupiter's daughter Minerva, the goddess of wisdom.
Unlike the earlier Archaic Triad, which was fairly typical of a trio of supreme divine beings, this grouping of a male god and two goddesses was highly unusual in ancient Indo-European religions. It is almost certainly derived from the Etruscan trio of Tinia, the supreme deity, Uni, his wife, and Menrva, their daughter and the goddess of wisdom.
Jupiter, Juno and Minerva were honored in temples known as Capitolia, which were built on hills and other prominent areas in many cities in Italy and the provinces, particularly during the Augustan and Julio-Claudian periods. Most had a triple cella. The earliest known example of a Capitolium outside of Italy was at Emporion in Spain. According to Ovid, Terminus also had a place there, since he had a shrine there before it was built and, as the god of boundary stones, refused to yield.
Although the word Capitolium (pl. Capitolia) could be used to refer to any temple dedicated to the Capitoline Triad, it referred especially to the temple on the Capitoline Hill in Rome known as aedes Iovis Optimi Maximi Capitolini ("the temple of the Best, Greatest, Capitoline Jupiter"). The temple was built under the reign of Lucius Tarquinius Superbus, the last King of Rome prior to the establishment of the Roman Republic. Although the temple was shared by Jupiter, Juno and Minerva, each deity had a separate cella, with Juno Regina on the left, Minerva on the right, and Jupiter Optimus Maximus in the middle. It included a podium and a tetrastyle (four column) pronaos (porch).
Another shrine (sacellum) dedicated to Jupiter, Juno Regina and Minerva was the Capitolium Vetus on the Quirinal Hill. It was thought to be older than the more famous temple of Jupiter Optimus Maximus on the Capitoline Hill, and was still a landmark in Martial's time, in the late 1st century.
The original three deities thus worshipped, now more commonly referred to as the Archaic Triad, were Jupiter, Mars and Quirinus. This structure was no longer clearly detectable in later times, and only traces of it could be identified from various literary sources and other testimonies.
Georg Wissowa, in his manual of the Roman religion, identified the structure as a triad on the grounds of the existence in Rome of the three flamines maiores, who carry out service to these three gods. He remarked that this triadic structure looks to be predominant in many sacred formulae which go back to the most ancient period and noted its pivotal role in determining the ordo sacerdotum, the hierarchy of dignity of Roman priests: rex sacrorum, flamen dialis, Flamen Martialis, flamen quirinalis and pontifex maximus in order of decreasing dignity and importance. He remarked that since such an order did no longer reflect the real influence and relationships of power among priests in the later times, it should have reflected a hierarchy of the earliest phase of Roman religion.
Wissowa identified the presence of such a triad also in the Umbrian ritual of Iguvium where only Iove, Marte and Vofionus are granted the epithet of Grabovius and the fact that in Rome the three flamines maiores are all involved in a peculiar way in the cult of goddess Fides.
However Wissowa did not pursue further the analysis of the meaning and function of the structure (which he called Göttersystem) he had identified.
Georges Dumézil in various works, particularly in his Archaic Roman Religion advanced the hypothesis that this triadic structure was a relic of a common Proto-Indo-European religion, based on a trifunctional ideology modelled on the division of that archaic society. The highest deity would thus be a heavenly sovereign endowed with religious, magic and legal powers and prerogatives (connected and related to the king and to priestly sacral lore in human society), followed in order of dignity by the deity representing braveness and military prowess (connected and related to a class of warriors) and lastly a deity representing the common human worldly values of wealth, fertility, and pleasure (connected and related to a class of economic producers). According to the hypothesis, such a tripartite structure must have been common to all Indoeuropean peoples on accounts of its widespread traces in religion and myths from India to Scandinavia, and from Rome to Ireland. However it had disappeared from most societies since prehistoric times, with the notable exception of India.
In Vedic religion the sovereign function was incarnated by Dyaus Pitar and later appeared split into its two aspects of uncanny and awe inspiring almighty power incarnated by Varuna and of source and guardian of justice and compacts incarnated by Mitra. Indra incarnated the military function and the twins Ashvins (or Nasatya) the function of production, wealth, fertility and pleasure. In human society the rajah and the class of the brahmin priests represented the first function (and enjoyed the highest dignity), the warrior class of the kshatriya represented the second function and the artisan and merchant class of the vaishya the third.
Similarly in Rome Jupiter was the supreme ruler of the heavens and god of thunder, represented on earth by the rex, king (later the rex sacrorum) and his substitute, the flamen dialis, the legal aspect of sovereignty being incarnated also by Dius Fidius, Mars was the god of military prowess and a war deity, represented by his flamen martialis; and Quirinus the enigmatic god of the Roman populus ("people") organised in the curiae as a civilian and productive force, represented by the flamen quirinalis.
Apart than from the analysis of the texts already collected by Wissowa, Dumezil stressed the importance of the tripartite plan of the regia, the cultic centre of Rome and official residence of the rex. As recorded by sources and confirmed by archeological data it was devised to lodge the three major deities Iupiter, Mars, and Ops, the deity of agricultural plenty, in three separate rooms.
The cult of Fides involved the three flamines maiores: they were carried to the sacellum of the deity together in a covered carriage and officiated with their right hand wrapped up to the fingers in a piece of white cloth. The association with the deity that founded divine order (Fides is associated with Iupiter in his function of guardian of the supreme juridical order) underlines the mutual interconnections among them and of the gods they represented with the supreme heavenly order, whose arcane character was represented symbolically in the hidden character of the forms of the cult.
The spolia opima were dedicated by the person who had killed the king or chief of the enemy in battle. They were dedicated to Jupiter in case the Roman was a king or his equivalent (consul, dictator or tribunus militum consulari potestate), to Mars in case he was an officer and to Quirinus in case he was common soldier. The sacrificial animals too were in each case the ones of the respective deity, i. e. an ox to Jupiter, solitaurilia to Mars and a male lamb to Quirinus.
Besides Dumézil analysed the cultual functions of the flamen quirinalis to better understand the characters of this deity. One important element was his officiating on the feriae of the Consualia aestiva ( of the Summer), which associated Quirinus to the cult of Consus and indirectly of Ops (Ops Consivia). Other feriae on which this flamen officiated were the Robigalia, the Quirinalia that Dumezil identifies with the last day of the Fornacalia, also named stultorum feriae because on that day the people who had forgot to roast their spelt on the day prescribed by the curio maximus for their own curia were given a last chance to make amends, and the Larentalia held in memory of Larunda. These religious duties show Quirinus was a civil god related to the agricultural cycle and somehow to the worship of Roman ancestry.
In Dumézil's view the figure of Quirinus became blurred and started to be connected to the military sphere because of the early assimilation to him of the divinised Romulus, the warring founder and first king of Rome. A coincident facilitating factor of this interpretation was the circumstance that Romulus carried with himself the quality of twin and Quirinus had a correspondence in the theology of the divine twins such the Indian Ashvins and the Scandinavians Vani. The resulting interpretation was the mixed civil and military, warring and peaceful personality of the god.
A detailed discussion of the sources is devoted by Dumézil to showing that they do not support the theory of an agrarian Mars. Mars would be invoked both in the Carmen Arvale and in Cato's prayer as the guardian, the armed protector of the fields and the harvest. He is definitely not a deity of agricultural plenty and fertility.
It is also noteworthy that according to tradition Romulus established the double role and duties, civil and military, of the Roman citizen. In this way the relationship between Mars and Quirinus became a dialectic one, since Romans would regularly pass from the warring condition to the civil one and vice versa. In the yearly cycle this passage is marked by the rites of the Salii, they themselves divided into two groups, one devoted to the cult of Mars (Salii Palatini, created by Numa) and the other of Quirinus (Salii Collini, created by Tullus Hostilius).
The archaic triad in Dumézil's view was not strictly speaking a triad, it was rather a structure underlying the earliest religious thought of the Romans, a reflection of the common Indoeuropean heritage.
This grouping has been interpreted as a symbolic representation of early Roman society, wherein Jupiter, standing in for the ritual and augural authority of the Flamen Dialis (high priest of Jupiter) and the chief priestly colleges, represents the priestly class, Mars, with his warrior and agricultural functions, represents the power of the king and young nobles to bring prosperity and victory through sympathetic magic with rituals like the October Horse and the Lupercalia, and Quirinus, with his source as the deified form of Rome's founder Romulus and his derivation from co-viri ("men together") representing the combined military and economic strength of the Roman people.
According to Georges Dumézil's trifunctional hypothesis, this division symbolizes the overarching societal classes of "priest" (Jupiter), "warrior" (Mars) and "farmer" or "civilian" (Quirinus). Though both Mars and Quirinus each had militaristic and agricultural aspects, leading later scholars to frequently equate the two despite their clear distinction in ancient Roman writings, Dumézil argued that Mars represented the Roman gentry in their service as soldiers, while Quirinus represented them in their civilian activities. Although such a distinction is implied in a few Roman passages, such as when Julius Caesar scornfully calls his soldiers quirites ("citizens") rather than milites ("soldiers"), the word quirites had by this time been dissociated with the god Quirinus, and it is likely that Quirinus initially had an even more militaristic aspect than Mars, but that over time Mars, partially through synthesis with the Greek god Ares, became more warlike, while Quirinus became more domestic in connotation. Resolving these inconsistencies and complications is difficult chiefly because of the ambiguous and obscure nature of Quirinus' cult and worship; while Mars and Jupiter remained the most popular of all Roman gods, Quirinus was a more archaic and opaque deity, diminishing in importance over time.