similar in etymology and semantics (dies, "daylight" and Dius, "daytime sky"), but differ linguistically. Wissowa considers the epithet Dianus noteworthy. Dieus is the etymological equivalent of ancient Greece's Zeus and of the Teutonics' Ziu (genitive Ziewes). The Indo-European deity is the god from which the names and partially the theology of Jupiter, Zeus and the Indo-Aryan Vedic Dyaus Pita derive or have developed. The Roman practice of swearing by Jove to witness an oath in law courts is the origin of the expression "by Jove!"—archaic, but still in use. The name of the god was also adopted as the name of the planet Jupiter; the adjective "jovial" originally described those born under the planet of Jupiter (reputed to be jolly, optimistic, and buoyant in temperament). Jove was the original namesake of Latin forms of the weekday now known in English as Thursday (originally called Iovis Dies in Latin). These became jeudi in French, jueves in Spanish, joi in Romanian, giovedì in Italian, dijous in Catalan, Xoves in Galician, Joibe in Friulian and Dijóu in Provençal. Major epithets The epithets of a Roman god indicate his theological qualities. The study of these epithets must consider their origins (the historical context of an epithet's source). Jupiter's most ancient attested forms of cult belong to the State cult: these include the mount cult (see section above note n. 22). In Rome this cult entailed the existence of particular sanctuaries the most important of which were located on Mons Capitolinus (earlier Tarpeius). The mount had two tops that were both destined to the discharge of acts of cult related to Jupiter. The northern and higher top was the arx and on it was located the observation place of the augurs (auguraculum) and to it headed the monthly procession of the sacra Idulia. On the southern top was to be found the most ancient sanctuary of the god: the shrine of Iuppiter Feretrius allegedly built by Romulus, restored by Augustus. The god here had no image and was represented by the sacred flintstone (silex). The most ancient known rites, those of the spolia opima and of the fetials which connect Jupiter with Mars and Quirinus are dedicated to Iuppiter Feretrius or Iuppiter Lapis. The concept of the sky god was already overlapped with the ethical and political domain since this early time. According to Wissowa and Dumézil Iuppiter Lapis seems to be inseparable from Iuppiter