Brythonic initial s becomes h in the 7th century, but this is unknown in Goidelic, e.g. Ir. salann, “salt,” W. halen, Cornish haloin, Bret, holenn; Lat. sē-men, Ir. sil, “seed,” W. hil. Initial v gives f in Goidelic in the course of the 7th century, whereas in Brythonic it appears as gu, gw, cf. Lat. vērus, Ir. fír, W., Bret. gwir. We may also mention that in Goidelic initial j and medial v disappear, e.g. Gaulish Jovincillus, W. ieuanc, “young,” Bret, iouank, Ir. óac, óc; W. bywyd, “food,” Ir. biad. Post-consonantic j in Brythonic sometimes gives -id (Mod. W. -ydd, Mod. Bret, -ez), e.g. Gaulish nevio-, novio-, O. Bret, nowid, W. newydd, Bret, nevez, Ir. núe. I.E. -kt and -pt both appear in Goidelic as -cht but in Brythonic as -ith, cf. Lat. septem, O. Ir. secht, W. seith, Bret. seiz.

We unfortunately know very little about the position of the stress in ancient Gaulish. According to Meyer-Lübke in place-names the penult was accented if the vowel was long, otherwise the stress lay on the preceding syllable, e.g. Augustodūnum, O. Fr. Ostedun, now Autun; Cataláunos (Châlons), Trícasses (Fr. Troyes), Bitúriges (Fr. Bourges). In Goidelic the stress, which is strongly expiratory, is always placed on the first syllable except in certain cases in verbs compounded with prepositional prefixes. In Old Welsh and Old Breton, on the other hand, the final syllable, i.e. the primitive penult, received the stress, but in both languages the stress was shifted in the middle period to the penultimate. The Goidelic dialects, like the Slavonic, distinguish between palatalized and nonpalatalized consonants, according as the consonant was originally followed by a front (e, i) or back vowel (a, o, u), a phenomenon which is entirely unknown to Brythonic.

Finally, the two groups differ radically in the matter of initial mutation or, as it is often called, aspiration. These mutations are by no means confined to initial consonants, as precisely the same changes have taken place under similar conditions in the interior of words. The Goidelic changes included under this head probably took place for the most part between the 5th and 7th centuries, whilst in Brythonic the process seems to have begun and continued later. It is easier to fix the date of the changes in Brythonic than in Goidelic, as a number of British names are preserved in lives of saints, and it is possible to draw conclusions from the shape that British place-names assumed in the mouths of the Anglo-Saxons. In Goidelic, we find two mutations, the vocalic and the nasal. Initial mutation only takes place between words which belong together syntactically, and which form one single stress-group, thus between article, numeral, possessive pronoun or preposition, and a following substantive; between a verbal prefix and the verb itself.

The relation in which the other Celtic dialects stand to this system will be mentioned below in dealing with the various languages. It will be noted from what has been said above that, with the exception of the different treatment of the labialized velar qv, and the nasal sonant ṇ, the features which differentiate the Brythonic from the Goidelic dialects first appear for the most part after the Romans had left Britain. At the beginning of the Christian era the difference between the two groups can only have been very slight. And Strachan has shown recently that Old Irish and Old Welsh agree in a very striking manner in the use of the verbal particle ro and in other syntactical peculiarities connected with the verb.

(i.) Goidelic.—The term Goidelic is used to embrace the Celtic dialects of Ireland, Scotland and the Isle of Man. In each case the national name for the speech is Gaelic (Ir. Gaedhlig, Scottish Gàidhlig, Manx Gailck), from Ir. Scottish Gaodhal, Gaedheal, Mid. Ir. Góedel, W. Gwyddel, “a Gael, inhabitant of Ireland or Scotland.” Old Irish may be regarded as the ancestor of Scottish and Manx Gaelic, as the forms of these dialects can be traced back to Old Irish, and there are practically no monuments of Scottish and Manx in the oldest period. Scottish and Irish may be regarded as standing to one another in much the same relation as broad Scottish and southern English. The divergences of Scottish and Manx from Irish will be mentioned below. The language of the Ogam inscriptions is the oldest form of Goidelic with which we are acquainted. Some 300 inscriptions have up to the present been discovered in this alphabet, the majority of them hailing from the south-west of Ireland (Kerry and Cork). In Scotland 22 are known, whilst in England and Wales about 30 have turned up. Most of the latter are in South Wales, but odd ones have been found in North Wales, Devon and Cornwall, and one has occurred as far east as Hampshire. The Isle of Man also possesses two. The letters in the oldest inscriptions are formed by strokes or notches scored on either side of the edge of an upright stone. Thus we obtain the following alphabet:—

This system, which was eked out with other signs, would seem to have been framed in the south-west of Ireland by a person or persons who were familiar with the Latin alphabet. Some of the inscriptions probably go back to the 5th century and may even be earlier. As illustrations of the simplest forms of Ogam inscriptions we may mention the following: Doveti maqqi Cattini, i.e. “(the stone) of Dovetos son of Cattinos”; Trenagusu Maqi Maqi-Treni is rendered in Latin Trenegussi Fili Macutreni hic jacit; Sagramni Maqi Cunatami, “(the stone) of Sagramnos son of Cunotamos”; Ovanos avi Ivacattos, “(the stone) of Ovanus descendant of Ivacattus.” It will be seen that in the oldest of these inscriptions q is still kept apart from k (c), and that the final syllables have not disappeared (cf. maqqi, O. Ir. maicc), but it appears certain that in Ogamic writing stereotyped forms were used long after they had disappeared in ordinary speech. Several stones contain bilingual inscriptions, but the key to the Ogam alphabet is supplied by a treatise on Ogamic writing contained in the Book of Ballymote, a manuscript of the late 14th century. It should be mentioned that the Welsh