First used by the Asenid rulers of the Second Bulgarian Empire, the particle is the abbreviation of theophoric name Ioan (John), which comes from the original Hebrew Yohanan, meaning "God has favored".

Io appeared in most documents (written or engraved), as issued by their respective chancelleries, since the countries' early history, but its frequency and relative importance among the princely attributes varied over time.

In more informal contexts, Romanians occasionally applied the title to benefactors or lieges from outiside the two countries, including John Hunyadi and George II Rákóczi.

As it entered more general use and its meaning was obscured, the title was gradually confounded with the first-person pronoun, Eu, and alternated with the royal we, Noi, until being finally replaced by it in the 19th century.

The ultimate origin of Io is with the Biblical Yohanan (Hebrew: יוֹחָנָן), a reference to the divine right, and, in the baptismal name "John", an implicit expression of thanks for the child's birth; the abbreviation is performed as with other nomina sacra, but appears as Ioan in Orthodox Church ectenia.

[4] Historian Radu G. Păun describes it specifically as a "theophoric name", which "served to highlight that princely power derives directly from God and not from an intermediary agency".

[5] Theologian Ion Croitoru argues that Io placed Wallachian and Moldavian Princes under the patronage of John the Evangelist, and that it doubles as a reference to their status as defenders of the Orthodox faith.

[10] Other scholars, beginning with Marin Tadin in 1977, argue that, though widely understood and vocalized as "John", Io was originally a misunderstanding replicated by Bulgarian and Wallachian scribes alike.

Early adaptations of Io may date back to Wallachia's creation as an independent polity: as noted by Nistor, Basarab I was known in neighboring Serbia as Ivanko, and therefore "John".

[10] Numismatist Traian Bița endorsed this interpretation by noting that some period coinage, mysteriously minted by a Prince only known as IWAN, may be Basarab's own issue.

[12] Similarly, historian A. Stănilă argues that it was Basarab himself who adopted the title as an homage to the Asenids, which included his in-law Ivan Alexander.

[14] Any explicit use of Io remains unattested until 1364, when it is included on Nicholas Alexander' epitaph, who (as Nistor notes) may have been directly inspired by the monogram of John Palaiologos.

[15] Under Vladislav's brother and successor Radu I, coins in Latin began featuring IONS as a translation of Ιω and contraction of Iohannes.

[17] Bița notes that these were found alongside coins only mentioning IWAN, and hypothesizes that they refer to Ivanko Dobrotitsa, the last man to ruler over the Dobrujan Despotate.

[10] Mircea's enemy, Vlad I, who took the throne in 1396, was the first to use a Slavonic Io on Wallachia's Greater Seal version (featuring the Wallachian eagle or raven).

[20] A version also appears in the Commentaries of Pope Pius II, which render Vlad the Impaler's 1462 letter to Mehmed the Conqueror in Latin translation.

This historical record is also noted for including references to the Eastern Carpathians and the Black Sea, which various historians read as a clue that, by 1390, Moldavia had ended its expansion.

The 1507 Missal put out by Hieromonk Makarije, which is regarded as the first printed work in the history of Romania, features dedications to three Wallachian Princes, all of them introduced as Io: Vlad the Impaler, Radu the Great, and Mihnea the Evil.

[44] However, as Nistor notes, by the time when boyar Ieremia Movilă took the Moldavian throne (1595–1600, 1600–1606), Io had declined into a mere "diplomatic formula", and was no longer read as the "attribute of legitimacy and sovereignty".

Early adaptations of Io in this new cultural context appear in the titles of Michael the Brave, who briefly accomplished his project to unite under one rule Wallachia, Moldavia, and the Principality of Transylvania.

[45] In letters he addressed to his nominal liege Emperor Rudolf, he maintains the titles Io and Voivode (also preserved in German-language copies), but omits the other claims, and styles himself "humble servant of Your Highness".

Their letter of supplication uniquely addressed Transylvanian Prince George II Rákóczi with the Voivodal Io, also adapting his name into Racolțea.

[49] Mihnea III, who emerged as Prince in 1658, during the latter stages of this conflict, used Neo-Latin on his shillings, introducing him as IO MICHAEL RAD D CVL TR PR.

[50] Neo-Latin variants continued to be featured on coins, as with 1660s Moldavian shillings issued by Eustratie Dabija, or IOHAN ISTRATDORVV; some also had the all-Latin rendition of Ιω as IO.

In 1882, writer Alexandru Macedonski compared the Hurezu murals with the self-styling used by commoners, as in: Eu Gheorghĭe al Petriĭ ("I Gheorghe son of Petru").

[61] Romanian-language documents issued by this Prince, as well as by his competitor Mihai Racoviță, have Slavonic introductions, which include Ιω;[62] to his Wallachian apologist Radu Popescu, Nicholas is known as Io Nicolae Alexandru voevod.

At that stage, a 1530s painting of Vlad Vintilă de la Slatina in the Great Lavra, at Mount Athos, was wrongly identified as, then retouched into, a portrait of John I Tzimiskes.

[75] Alexandru Ioan Cuza, elected in 1859 as the first Domnitor to rule over both countries (the "United Principalities"), used a transitional mixture of Latin and Cyrillic (Нoi Alecsandru Joan 1.)

[78] During the 1860s and '70s, a period which resulted in the consolidation of union as the "Kingdom of Romania", the forgotten origins of Io became the object of scrutiny by historical linguists; this began in 1863 with an overview by Bogdan Petriceicu Hasdeu.

In April of that year, Alexandru Papiu Ilarian, emphasizing the need to Romanianize this foreign arrival, proposed in Parliament that he be titled as Ioan Carol.