The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
Before the faith in Christ was obtained, no other than twelve letters were used, namely, a, e, i, o, b, d, g, l, m, n, r, s. After the coming of the faith, sixteen were put in use, then the art of the twelve letters was lost, nor is there at present any one that knows it, except from conjecture. After the coining of Taliesin eighteen letters were used; and it was according to the art of the system of eighteen that O I U was appointed for the Name of God. Before that arrangement it was O I O according to the sixteen. After the time of Taliesin the use of twenty letters was obtained, 3 which continued until the time of Geraint the
p. 68 p. 69
[paragraph continues] Blue Bard, who made an arrangement of twenty-four letters. After that, from reasoning to reasoning, the Bards improving the Alphabet, increased the number to thirty-eight on wood; but in black and white 1 no other than twenty-four were used.
67:3 p. 67
The language of twenty letters is in Awen--Taliesin.
69:1 p. 68 I.e. in writing.