The Brythonic languages (or Brittonic languages) form one of the two branches of the Insular Celtic language family, the other being Goidelic. The name Brythonic is derived from the Welsh word Brython, meaning an indigenous Briton as opposed to an Anglo-Saxon or Gael. The Brythonic branch is also referred to as P-Celtic because the Brythonic reflex of the Proto-Indo-European phoneme *k is p as opposed to the Goidelic c. Such nomenclature usually implies an acceptance of the P-Celtic hypothesis rather than the Insular Celtic hypothesis (for a discussion, see Celtic languages).
Other major characteristics include:
The major Brythonic languages are Welsh and Breton, both of which survive as community languages today. The Cornish language died out at the end of the eighteenth century, but attempts at reviving it started in the 20th century and are ongoing. Also notable are the extinct language Cumbric, and possibly the extinct Pictish (although the late Kenneth H. Jackson argued during the 1950s, from some of the few remaining examples of Pictish, that Pictish was a non-Indo-European language, the majority of modern scholars of Pictish do not agree).

the treatment of -m, -n as -am, -an.
initial s- followed by a vowel was changed to h-

• Irish sean "old", sior "long", samail "similar"
  Breton hen, hir, heñvel
  Brythonic retains original nasals before -t
  
  
  
  • Breton kant "hundred" vs. Irish cead
    sp, sr, sv/sw became f, fr, chw
    
    
    
    • *swero "toy, game" became Welsh chwarae and Breton c'hoari
      *srokna "nostril" became W ffroen and Br froen.
      all other initial s- fell before consonants
      
      
      
      • smero became W mwyar, Br mouar "fruit"
        slemon became W llyfn, Br levn "smooth"
        v became gw where in Goidelic it is f
        
        
        
        • vindos "white" became W gwenn
          vassos "servant, young man" became W gwas
          double plosives transformed into spirants: pp, cc, tt became f, ch (c'h), th (z) before a vowel or liquid
          
          
          
          • cippus > Br kef "tree trunk", W cyff
            cattos > Br kaz, W cath
            bucca > Br boc'h, W boch
            single voiceless plosives and voiced d, b, and m in an intervocalic position became soft spirants
            
            
            
            • Welsh dd[ð], th[θ], f [v]
              Breton z, v History and origins
              The principal legacy left behind in those territories from which the Brythonic languages were displaced is that of toponyms (place-names). There are many Brythonic place-names in lowland Scotland and in the parts of England where it is agreed that substantial Brythonic speakers remained (Brythonic names, apart from those of the former Romano-British towns, are scarce over most of England). Names derived (sometimes indirectly) from Brythonic include London, Penicuik, Perth, Aberdeen, York, Dorchester, Dover and Colchester. Brythonic elements found in England include bre- and bal- for hills, and carr for a high rocky place, while some such as coomb(e) for a small deep valley and tor for a hill are examples of Brythonic words that were borrowed into English. Others reflect the presence of Brythons, such as Dumbarton - from the Scottish Gaelic Dùn Breatainn meaning "Fort of the Britons", or Walton (several) meaning a 'tun' or settlement where Welsh/Brythons still lived.
              It is generally accepted that linguistic effects on English were lexically rather poor aside from toponyms, consisting of a few domestic words, which may include hubbub, peat, bucket, crock, noggin, gob (cf. Gaelic gob), nook; and the dialectal term for a badger, i.e. brock (cf. Welsh broch, and Gaelic broc). Arguably, the use of periphrastic constructions (using auxiliary verbs like do and be) in the English verb (which is more widespread than in the other Germanic languages) is traceable to Brythonic influence.
              Some researchers (Filppula et al., 2001) argue that English syntax reflects more extensive Brythonic influences. For instance, in English tag questions, the form of the tag depends on the verb form in the main statement (aren't I?, isn't he?, won't we? etc). The German nicht wahr? and the French n'est-ce pas?, by contrast, are fixed forms which can be used with almost any main statement. It has been claimed that the English system has been borrowed from Brythonic, since Welsh tag questions vary in almost exactly the same way. This view is far from being generally accepted, though.
              Far more notable, but less well known, are the many Brythonic influences on Scottish Gaelic. Like English, periphrastic constructions have come to the fore, but to a much greater degree. Scottish Gaelic contains a number of apparently P-Celtic loanwords, but as there is a far greater overlap in terms of Celtic vocabulary, than with English, it is not always possible to disentangle P and Q Celtic words. In particular, the word srath (anglicised as "Strath") is a native Goidelic word, but its usage appears to have been modified by the Brythonic cognate whose meaning is slightly different.