The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
The palm Coelbren is that where twenty-four are cut on small dice, that is, inasmuch as each die has six sides, and a letter on each side, there will be on the four dice twenty-four letters, besides what may be obtained otherwise, when the die is reversed, in order to show a different letter.
Thus, and others, being reversed, will make , &c. that is, a, â, e, ê, ng, c, d, y, y, m, turned upside down will make w, ŵ, p, ff, ngh, d, dd, l, ll, w. By holding some of these in the palm of the hand, and putting them together in the presence of a man of secrecy, dumb conversation can be carried on.