The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
Before the time of Beli the Great, 2 son of Manogan, there were only ten letters, which were called the ten signs, namely, a, p, c, e, t, i, l, r, o, s. After that m, and n, were invented; and after that four others, and they were made into sixteen by the divulgation, and under the proclamation of country and nation. After the coming of the faith in Christ, two other letters, namely u and d. In the time of king Arthur 3 there were introduced twenty primary letters, as at present, by the counsel of Taliesin, the chief of Bards, and domestic Bard of Urien Rheged. 4 Under the system of the eighteen were arranged O. I. U. which is the
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unutterable Name of God; whereas previous to that arrangement it was O. I. O. according to the sixteen. Of the principal signs there are not, to this day, more than twenty letters, or twenty signs. Geraint the Blue Bard appointed twenty-four letters, as it is at present; but the four are auxiliaries. After that, through the argumentative consideration of Bards, and Teachers who were chair Bards, there were brought into use and privilege, by the improvement of the Coelbren, thirty-eight letters on wood; but there are in black and white 1 only the twenty-four signs.
57:2 p. 56 He was the father of the celebrated Casswallawn or Cassivelaunus, who opposed the Roman invasion.
57:3 Arthur was elected pendragon of the Britons about A. D, 517, and died A, D. 552.
57:4 p. 57 Several of Taliesin's poems to Urien Rheged are printed in the 1st vol. of the Myvyrian Archaiology.
59:1 p. 58 I e. in writing.