William Ridgeway, Edmund C. Quiggin and William J. Gruffydd, 1911 Encyclopædia Britannica, Volume 5… (1911)
“ Celtic Languages Introduction.—The Celtic languages form one group of the Indo-European family of languages. As might be expected from their geographical distribution, they hold a position between the Italic and Teutonic groups. They are distinguished from these and other branches of the family by certain well-marked characteristics, the most notable of which are the loss of initial and inter-vocalic p, cf. Ir. athair with Lat. pater; Ir. lān, “full,” Welsh llawn, Breton leun, with Lat. plenus; Gaulish are-, “beside,” Ir. ar. Welsh, Breton ar, with Gr. περί, παρά ”
