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§ 145 Most compound adjectives, as hy-glyw, hy-glod, e-ang, ffrwyth-lon, melỿs-lais, etc. But when the second element is an adj. which may take ‑i̯on, the ending is sometimes affixed to the compound; thus claer-wynnẏon Ỻ.A. 92 ‘bright’, gloyw-ẟuon do. 93 ‘glossy-back’; glas-feinion D.G. 87 ‘green and slender’, tal- gryfi̯on Ezek. iii 7 “of an hard forehead”.

Many adjectives have substantival plurals used partly as abstract nouns as uchelion Gr.O. 120 ‘heights’, but chiefly to denote classes of persons; the sg. is also in some cases substantival. The pl. is formed either by affection or by adding ‑i̯on or ‑i̯aid, Ml. W. ‑ẏon, ‑ẏeit; the latter is used for persons only, and causes the same penult, affection as ‑i̯on, except in late formations. Thus caeth ‘slave’ pl. keith, Mn. W. caith L.G.C. 63, or Ml. W. keithẏeit or Mn. W. caethion; byddar ‘deaf’ pl. byddair, later formation byddariaid; balch ‘proud’ pl. beilch 1334 l. 46, beilchion, beilchiaid; truan ‘wretch’ pl. truain, trueinion, trueiniaid; gwan ‘weak’, pl. gweinẏon  i 220b, gweinẏeit  1196, Mn. W. gweiniaid; dall ‘blind’ pl. deillion, deilliaid.
 * Ar ol y ferch ar ŵyl Fair
 * O gloi’r bedd e glyw’r byddair.—T.A., . ii 83.

‘The deaf hear [the lamentations] for the maid on Lady Day at the closing of the grave.’
 * A’i lun gwrol yn gorwedd
 * Ef a wna i’r beilch ofni’r bedd.—T.A., 14975/107.

‘Since his manly form lies [in it], he makes the proud fear the grave.’
 * Be chwilid pob ach aliwn,
 * Bylchau’n ach beilchion a wn.—TA., 14966/277.

‘If every alien pedigree were examined I know gaps in the pedigree of proud ones.’
 * A phlaid o feilchiaid a fydd.—D.E., 100/249.

‘And there will be a company of the proud.’

. gweini̯aid is often used adjectivally in Mn. W., as rhai gweiniaid 1 Cor. ix 22; on the other hand gweinion is often a noun