Celtic toponymy

Celtic toponymyis the study of place names wholly or partially ofCelticorigin. These names are found throughout continental Europe,Britain,Ireland,Anatoliaand, latterly, through various other parts of the globe not originally occupied byCelts.
TheProto-Indo-European languagedeveloped into various daughter languages, including theProto-Celtic language.
In Proto-Celtic ("PC"), the Proto-Indo-European ("PIE") sound *pdisappeared, perhaps through an intermediate *ɸ. It is a common point between all the Celtic languages. Examples : Latinpater"father", but Gaulish*atir/ater(atrebo,dativplural), (Old) Irishathair/athir.
After that, languages derived from Proto-Celtic changed PC *kinto either *por *k(see:P-Celtic and Q-Celtic languages). In P-Celtic languages, PC *kchanged into *p. In Q-Celtic dialects it developed into /k/.
P-Celtic languages include the ContinentalGaulish languageand theBrittonicbranch of Insular Celtic.Common Brittonicis the ancestor ofWelsh,CornishandBreton.
Ancient Q-Celtic languages include the ContinentalCeltiberianand theGoidelicbranch of Insular Celtic. Goidelic is the ancestor of the Gaelic languagesIrish,Scottish GaelicandManx.
Examples : PIE*kʷetwóres"four" >
Most of the main cities in France have a Celtic name (the original Gaulish one or the name of the Gaulish tribe).
From Celtic*alisa, s.f., 'alder'. (Compare the modernGermanErlenbach) andOld High German(OHG)aha, s.n., 'flowing water'.
Switzerland, especially theSwiss Plateau, has many Celtic (Gaulish) toponyms. This old layer of names was overlaid with Latin names in theGallo-Romanperiod,and, from the medieval period, withAlemannic Germanand Romancenames.
For some names, there is uncertainty as to whether they are Gaulish or Latin in origin. 
In some rare cases, such asFrick, Switzerland, there have even been competing suggestions of Gaulish, Latin and Alemannic etymologies.
Examples of toponyms with established Gaulish etymology:
The main survey of Celtic place-names in this region is byRichard CoatesandAndrew Breeze.Evidence for a Celtic root to place names inEnglandis widely strengthened by early monastic charters, chronicles and returns: examples relate to Leatherhead and Lichfield. To describe a place as of the Celts, the Old EnglishwealhbecomingWal/Wall/Welshis often used. This was the main Germanic term for Romano-Celtic peoples, such as theBritons. Such names are a minority, but are widespread across England. For example, a smattering of villages around the heart and east ofThe Fenshint at this:West Walton,Walsoken, and theWalpolesindicate their continued presence. NearbyWisbech,King's LynnandChatterishave Celtic topographical elements.
The post-6th century AD Brittonic languages of Northern England and Scotland wereCumbricandPictish. Cumbric place-names are found in Scotland south of the River Forth,while Pictish names are found to the north.
The vast majority of placenames inWales(part of the United Kingdom) are eitherWelshor anglicized Welsh.
The vast majority of placenames inCornwallare eitherCornishor anglicized Cornish. For examples, seeList of places in Cornwall.
The vast majority of placenames in the west ofBrittany(part ofFrance) are eitherBretonor derived from Breton. For examples, seeCategory:Populated places in Brittany.
Place names in England derived partly or wholly from Goidelic languages include:
Furthermore, some non-Goidelic place-names in mainly Northern and Midland England reference Irish personal names, due toNorse-Gaelicsettlement Britain during the 10th century.
Place names that directly reference the Irish includeIrby,Irby upon Humber,IrebyandIreleth. Place names withScot-or similar, such asScothernin Lincolnshire orScottonin the North Riding of Yorkshire, may refer to Gaelic speakers from Scotland or Ireland, since Old EnglishScottasoriginally had connotations of Irish Gaels.
The vast majority of placenames inIrelandare anglicizedIrish languagenames.
The majority of placenames in theHighlandsofScotland(part of theUnited Kingdom) are eitherScottish Gaelicor anglicized Scottish Gaelic. Gaelic-derived placenames are very common in the rest of mainlandScotlandalso.Pictish-derived placenames can be found in the northeast, while Brythonic-derived placenames can be found in the south.
Many placenames on theIsle of Man(aCrown dependency) areManxor anglicised Manx, although there are also many Norse-derived place names.