The Celtic languages are a group within the larger Indo-European language family. They are believed to have developed from a language called Proto-Celtic, which is only a theory. The word "Celtic" was first used to describe this language group by Edward Lhuyd in 1707. He was influenced by Paul-Yves Pezron, who connected the Celts mentioned by ancient writers to the Welsh and Breton languages.

During the first millennium BC, Celtic languages were spoken across much of Europe and central Anatolia. Today, they are mainly spoken in the northwestern part of Europe and in some communities outside of Europe. There are six Celtic languages still in use: Breton, Irish, Scottish Gaelic, and Welsh, which have been spoken continuously, and Cornish and Manx, which were once lost but have been brought back. These languages are minority languages in their countries, but people are working to keep them alive. Welsh is an official language in Wales, and Irish is an official language in Ireland and the European Union. Welsh is the only Celtic language not considered at risk of disappearing by UNESCO. Cornish and Manx were once extinct but now have many speakers who are learning them as a second language.

Irish, Manx, and Scottish Gaelic are part of the Goidelic languages. Welsh, Cornish, and Breton are part of the Brittonic languages. All of these are called Insular Celtic languages because Breton, the only Celtic language spoken in continental Europe, came from people who moved from Britain. Some Celtic languages that are no longer spoken include Galatian, Lepontic, Gaulish, Celtiberian, and Gallaecian. The last two form a group called Hispano-Celtic. There is no agreement on how to divide the Celtic language family. Traditionally, they are split into P-Celtic and Q-Celtic. However, Gaulish is more closely related to Insular Celtic than to Celtiberian. This connection is called Nuclear Celtic.

The Celtic languages have a long history of written works. The oldest known Celtic writing is from the 6th century BC and includes Lepontic inscriptions found in the Alps. Early writings in Europe used scripts like Italic and Paleohispanic. Between the 4th and 10th centuries, Irish and Pictish were written in a script called Ogham. Over time, Latin script became the standard for all Celtic languages. Welsh has had a continuous written tradition since the 6th century AD.

Living languages

SIL Ethnologue reports that there are six living Celtic languages. Four of these languages still have many native speakers. These include the Goidelic languages (Irish and Scottish Gaelic, both developed from Middle Irish) and the Brittonic languages (Welsh and Breton, developed from Common Brittonic). The other two languages, Cornish (Brittonic) and Manx (Goidelic), no longer have native speakers. Their last known native speakers lived in 1777 and 1974, respectively. In the 2000s, efforts to revive these languages helped adults and children begin learning and speaking them again.

• Beurla Reagaird: a language spoken by Highland travelers
• Shelta: a language that mixes Irish and a type of English spoken in Ireland (about 86,000 speakers in 2009)

Classification

Celtic languages are usually divided into several groups:

• Continental Celtic languages, a group that is now completely extinct and was spoken in mainland Europe.
• Insular Celtic languages, spoken in the British Isles and Brittany today.
• Hispano-Celtic languages, a subgroup of Continental Celtic, including Celtiberian and Gallaecian. Celtiberian, also called Eastern or Northeastern Hispano-Celtic, was spoken in the ancient Iberian Peninsula, in the eastern part of Old Castile and south of Aragon (modern provinces: Segovia, Burgos, Soria, Guadalajara, Cuenca, Zaragoza, and Teruel). The connection between Celtiberian and Gallaecian, spoken in northwest Iberia, is not certain. Gallaecian, also known as Western or Northwestern Hispano-Celtic, was spoken in the northwest of the peninsula (modern Northern Portugal and the Spanish regions of Galicia, Asturias, and northwestern Castile and León).

• Lepontic, part of the Continental Celtic group, is the oldest known Celtic language (from the 6th century BC). It was spoken in Switzerland and northern-central Italy. Coins with Lepontic writing have been found in Noricum and Gallia Narbonensis.
• Gaulish languages, once spoken in a wide area from Belgium to modern-day Turkey, but all are now extinct since the 7th–8th centuries AD. Transalpine Gaulish was spoken in modern France, Switzerland, and parts of Germany. Cisalpine Gaulish, a language proposed based on inscriptions, is different from Transalpine Gaulish and closer to Lepontic. It was spoken in northern Italy until the 1st century BC. Galatian was spoken in central Anatolia and may have lasted until the 6th century AD.
• Brittonic, spoken in Great Britain and Brittany. It includes the living languages Breton, Cornish, and Welsh, and the lost languages Cumbric and possibly Pictish. Before the arrival of Scotti on the Isle of Man in the 9th century, the island may have spoken a Brythonic language. Some theories suggest a Brittonic Ivernic language existed in Ireland before Goidelic speech, but this idea is widely rejected.
• Pritenic, spoken in most of Scotland, is a hypothetical subgroup of Insular Celtic from which the Pictish language may have developed. It may be related to Brittonic but is distinct. Some earlier theories suggested Pictish was a pre-Indo-European language, but this is not widely supported.
• Goidelic, which includes the modern languages Irish, Manx, and Scottish Gaelic, spoken in modern Scotland, Ireland, and the Isle of Man.

Scholars have debated how to classify Celtic languages because there is not much primary source data. Some scholars (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) argue that the main distinction is between Continental Celtic and Insular Celtic, claiming that differences between Goidelic and Brittonic languages developed after these groups separated from Continental Celtic. Other scholars (such as Schmidt 1988) focus on the distinction between P-Celtic and Q-Celtic languages, based on the replacement of the letter Q with P in some words. Most Gallic and Brittonic languages are P-Celtic, while Goidelic and Hispano-Celtic languages are Q-Celtic. P-Celtic languages (also called Gallo-Brittonic) are sometimes seen as a central group of innovators compared to the more conservative Q-Celtic languages. According to Ranko Matasović in his 2009 Etymological Dictionary of Proto-Celtic, "Celtiberian… is almost certainly an independent branch on the Celtic family tree, one that became separated from the others very early."

The Breton language is Brittonic, not Gaulish; though it may have some influence from Gaulish, it was introduced from southwestern Britain after the Roman era. In the P/Q classification system, Gaelic was the first language to split from Proto-Celtic. Some scholars see its features as archaic, while others believe they are also found in Brittonic languages. In the Insular/Continental classification system, the split between Gaelic and Brittonic is seen as occurring later.

The division of Celtic into these groups likely happened around 900 BC, according to Gray & Atkinson, though estimates range from 1200 to 800 BC. A controversial study by Forster & Toth included Gaulish and suggested the split occurred much earlier, around 3200 BC, but this is disputed. The early Celts were often linked to the Urnfield, Hallstatt, and La Tène cultures, though the connection between language and culture is now considered weaker.

Scholars continue to debate whether the Insular Celtic hypothesis or the P-/Q-Celtic hypothesis is more accurate. Since the 1970s, the division into Insular and Continental Celtic has been more widely accepted, but the P-/Q-Celtic theory gained support in the 1980s due to discoveries like the Larzac lead inscription, which showed a shared phonetic change. Some experts argue that the Insular Celtic hypothesis is stronger because of developments in grammar and syntax in Irish and British Celtic, while others consider the P-/Q-Celtic division less important.

When discussing only modern Celtic languages, "Q-Celtic" is the same as "Goidelic," and "P-Celtic" is the same as "Brittonic," since no Continental Celtic language has living descendants.

The way the family tree of Celtic languages is organized depends on which theory is used:

"Insular Celtic hypothesis"
"P/Q-Celtic hypothesis"

Eska evaluates evidence supporting a tree based on shared innovations, though some features may be regional rather than genetic. It is likely that Celtiberian split off before Cisalpine Celtic, but this is not certain. The unity of Gaulish, Goidelic, and Brittonic is more secure. Schumacher (2004) suggested this grouping may be genetic, based on shared changes in sentence structure. Eska sees Cisalpine Gaulish as closer to Lepontic than to Transalpine Gaulish.

Eska believes the most probable division is between Transalpine–Goidelic–Brittonic and Transalpine Gaulish, due to more innovations in Insular Celtic and limited contact between Insular and P-Celtic languages. However, if other explanations (like an SOV substratum language) are valid, P-Celtic could form a valid

Possible members

Several poorly documented languages may have been Celtic; often referred to as Para-Celtic:

• Noric is a language not fully confirmed, found in modern Austria and Slovenia through some inscriptions. It was sometimes called the language of the Eastern Celts, spoken in areas now including the Czech Republic, Serbia, Hungary, and Slovakia around the 2nd century AD. Inscriptions in Noric use the Old Italic alphabet, and they are unique because writing traditions in northern Italy had stopped by the late 1st century BC. Scholars are still unsure if this language existed, as only two inscriptions are available for study.

• Ancient Belgian is a language not fully confirmed, spoken in the region of Belgica. Most linguists believe it was Celtic, though some suggest it may have been Germanic or part of a different Indo-European group. Other theories link it to the Tyrsenian language family.

• Camunic was an extinct language spoken in the Central Alps around the first millennium BC in the Val Camonica and Valtellina valleys. Some recent studies suggest it may have been Celtic.

• Ivernic is a P-Celtic language once spoken in Ireland by the Iverni tribe, proposed by T. F. O'Rahilly. However, this theory is not widely accepted by experts.

• Ligurian was spoken along the Northern Mediterranean Coast, covering parts of southeast France, northwest Italy, Tuscany, Elba, and Corsica. Xavier Delamarre suggests it was a Celtic language similar to Gaulish. Scholars continue to debate whether it was Celtic (based on inscriptions) or Para-Celtic (based on names).

• Lusitanian was spoken in western Iberia, between the Douro and Tagus rivers (modern Portugal and Spain). It is known from only five inscriptions and some place names. It is an Indo-European language, and some scholars think it may have been a Para-Celtic language that developed alongside Celtic or shared similarities with Tartessian and Gallaecian. Others believe it may be related to Old Gallo-Italic or Old European. Ellis Evans suggests Gallaecian and Lusitanian were the same language, both P-Celtic.

• Rhaetic was spoken in central Switzerland, Tyrol in Austria, and the Alpine regions of northeast Italy. It is known from a few short inscriptions written in two forms of the Etruscan alphabet. Scholars are unsure of its classification, as it includes elements that resemble Etruscan, Indo-European, and other unknown languages. Howard Hayes Scullard once argued it was Celtic, but most scholars now believe it was a Tyrrhenian language closely related to Etruscan.

• Tartessian was spoken in southwest Iberia, mainly in southern Portugal and southwest Spain. It is known from 95 inscriptions, with the longest having 82 readable signs. John T. Koch suggests it was a Celtic language.

Characteristics

Although the Celtic languages differ in many ways, they share many similarities.

• Consonant changes (only in Insular Celtic languages)
• Inflected prepositions (only in Insular Celtic languages)
• Two grammatical genders (only in modern Insular Celtic languages; Old Irish and Continental Celtic languages had three genders, though Gaulish may have combined the neuter and masculine in later forms)
• A vigesimal number system (counting by twenties) Cornish hwetek ha dew ugens "fifty-six" (literally "sixteen and two twenty")
• Verb–subject–object (VSO) word order (probably only in Insular Celtic languages)
• An interaction between the subjunctive, future, imperfect, and habitual tenses, leading to some tenses and moods being replaced
• An impersonal or autonomous verb form used as a passive or intransitive Welsh dysgaf "I teach" vs. dysgir "is taught, one teaches" Irish múinim "I teach" vs. múintear "is taught, one teaches"
• No infinitives, replaced by a quasi-nominal verb form called the verbal noun or verbnoun
• Frequent use of vowel changes as a way to show grammatical changes, such as forming plurals or verb stems
• Use of preverbal particles to indicate subordination or the purpose of a clause, with mutation-distinguished subordinators/relativizers for negation, questioning, and sometimes affirmative statements
• Pronouns placed between particles and verbs
• No simple verb for the imperfective "have" process, with possession shown through a structure like BE + preposition Cornish Yma kath dhymm "I have a cat" (literally "there is a cat to me") Welsh Mae cath gyda fi "I have a cat" (literally "a cat is with me") Irish Tá cat agam "I have a cat" (literally "there is a cat at me")
• Use of periphrastic constructions to express verb tense, voice, or aspect
• A distinction between two forms of the BE verb, traditionally called substantive (or existential) and copula
• A two-part system for demonstrative words
• Pronouns added at the end of verbs, called confirming or supplementary pronouns
• Use of singular forms or special counted nouns, and a suffix to make singular forms from plurals when older singulars no longer exist

The similarity in vocabulary among Celtic languages is clear, especially in pronunciation. Phonetic differences between languages often result from regular sound changes, such as /b/ becoming /v/ or disappearing.

The table below shows words in modern Celtic languages that were inherited directly from Proto-Celtic, as well as some old Latin borrowings found in all daughter languages. Welsh, Breton, and Cornish often match more closely with each other than with Irish, Scottish Gaelic, and Manx. For more comparisons, refer to the Swadesh list for Celtic.

Borrowings from Latin

Article 1 of the Universal Declaration of Human Rights: All human beings are born free and equal in dignity and rights. They are given reason and conscience and should act toward one another in a spirit of brotherhood.

• Irish: Saolaítear gach duine den chine daonna saor agus comhionann i ndínit agus i gcearta. Tá bua an réasúin agus an choinsiasa acu agus ba cheart dóibh gníomhú i dtreo a chéile i spiorad an bhráithreachais.
• Manx: Ta dagh ooilley pheiagh ruggit seyr as corrym ayns ard-cheim as kiartyn. Ren Jee feoiltaghey resoon as cooinsheanse orroo as by chair daue ymmyrkey ry cheilley myr braaraghyn.
• Scottish Gaelic: Tha gach uile dhuine air a bhreith saor agus co-ionnan ann an urram 's ann an còirichean. Tha iad air am breith le reusan is le cogais agus mar sin bu chòir dhaibh a bhith beò nam measg fhèin ann an spiorad bràthaireil.
• Breton: Dieub ha par en o dellezegezh hag o gwirioù eo ganet an holl dud. Poell ha skiant zo dezho ha dleout a reont bevañ an eil gant egile en ur spered a genvreudeuriezh.
• Cornish: Genys frank ha par yw oll tus an bys yn aga dynita hag yn aga gwiryow. Enduys yns gans reson ha kowses hag y tal dhedha omdhon an eyl orth y gila yn spyrys a vrederedh.
• Welsh: Genir pawb yn rhydd ac yn gydradd â'i gilydd mewn urddas a hawliau. Fe'u cynysgaeddir â rheswm a chydwybod, a dylai pawb ymddwyn y naill at y llall mewn ysbryd cymodlon.