Gallo was originally spoken in the Marches of Neustria, an area now corresponding to the border lands between Brittany, Normandy, and Maine.

Gallo was a shared spoken language among many of those who took part in the Norman conquest of England, most of whom originated in Upper (i.e. eastern) Brittany and Lower (i.e. western) Normandy, and thus had its part, together with the much bigger role played by the Norman language, in the development of the Anglo-Norman variety of French which would have such a strong influence on English.

Gallo continued as the everyday language of Upper Brittany, Maine, and some neighbouring portions of Normandy until the introduction of universal education across France, but is spoken today by only a small (and aging) minority of the population, having been almost entirely superseded by standard French.

There is some limited mutual intelligibility with adjacent varieties of the Norman language along the linguistic frontier and with Guernésiais and Jèrriais.

The clearest linguistic border is that distinguishing Gallo from Breton, a Brittonic Celtic language traditionally spoken in the western territory of Brittany.

As of 1980[update], Gallo's western extent stretches from Plouha (Plóha), in Côtes-d'Armor, south of Paimpol, passing through Châtelaudren (Châtié), Corlay (Corlaè), Loudéac (Loudia), east of Pontivy, Locminé (Lominoec), Vannes, and ending in the south, east of the Rhuys peninsula, in Morbihan.

[4] In south Lower Normandy and in the west of Pays de la Loire it is often referred to as patois,[5] though this is a matter of some contention.

Henriette Walter conducted a survey in 1986 which showed that just over 4% of Gallo speakers in Côtes-d'Armor had ever used the term, and a third of them found it "had quite a pejorative connotation".

[8] The term britto-roman was coined by the linguist Alan-Joseph Raude in 1978 to highlight the fact that Gallo is "a Romance variety spoken by Bretons".

Gallo has not just borrowed words from Breton, but also aspects of grammar; the use of the preposition pour as an auxiliary verb is said to be of Celtic origin.

Following the Migration Period,[14] these two cities, as well as regions to the east of the Vilaine, including the town Vannes,[18] fell under Frankish rule.

Historically, France has been a nation with a high degree of linguistic diversity matched with relative tolerance, that is until the French Revolution.

Under the Third Republic, public education became universal and mandatory in France, and was conducted exclusively in French; students who spoke other languages were punished.

Shortly before World War II, the Regional Federation of Bretagne introduced the idea of rejuvenating Gallo's presences in schools.

Gallo and vernacular French share a sort of continuum, so speakers may have difficulty determining exactly which language they are speaking.

[21] Many people speak Gallo while using a considerable amount of French words and phrases, thus confounding the language question further.

The roots of written Gallo literature are traced back to Le Livre des Manières written in 1178 by Étienne de Fougères, a poetical text of 336 quatrains and the earliest known Romance text from Brittany, and to Le Roman d'Aquin, an anonymous 12th century chanson de geste transcribed in the 15th century but which nevertheless retains features typical of the mediaeval Romance of Brittany.

Gallese legends frequently address recurring characters, such as Gargantua and Morgan le Fay, or questions of how the countryside was created.

In the 19th century, oral literature was collected by researchers and folklorists such as Paul Sébillot, Adolphe Orain, Amand Dagnet and Georges Dottin.

Some words do not obey the rule, such as pátre and mátre, which have become pere [peʁ] and mere [meʁ] in practically all of Upper Brittany, while [pəʁ] and [məʁ] are only heard in the center-west.

[30] Like all langues d'oïl, Gallo underwent the vowel shift known as Bartsch's law, according to which the Latin [a] in open stressed syllables, when preceded by a palatal consonant, became ie, as in cápra, which became chieuvr.

Some terms, however, are influenced by neighboring langues d'oïl, and astour [astuʁ] ("now", from Latin hóra) is becoming [astœʁ] in eastern Upper Brittany.

In the south of Loire-Atlantique, thanks to contact with Poitevin, [ɔ] is common, and guernol [gɛʁnɔl] and parto [paʁtɔ] are heard instead of guernouille [gɛʁnuj] and partout [paʁtu].

Diphthongs in Gallo generally use the semi-vowels [w] and [j], more rarely [ɥ]: [wa], [wə], [wi], [aw], [ja], [ju], [aj], [ej], [ɛ̃i], [ɥi], [ɥɛ̃], [ɥə], etc.

If the orthography of French was used, the word could be written in countless ways: maï, maye, maille, mèï, mey, meille, ma, mé, mè, etc.

It proposed using French spelling as a baseline and adjusting it to fit Gallo’s unique phonetic features, such as using lh to indicate palatalization and ë to represent schwa.

Regional differences were less pronounced during the Medieval era, meaning ELG’s spelling choices are based on a more standardized form.

The system is the result of five years' research throughout Upper Brittany, and takes its name from the brochure presenting it to the public: Nostre lenghe aneit ("our language today").

[33] The Vantyé spelling system was developed again by the Bertègn Galèzz association in the early 1980s, and is notable for its attempt to be closer to Breton.

[34] In MOGA, [lj] is written lh, and ñ is used in place of n when the preceding consonant should not be nasal: diñra is pronounced [dinʁa].