The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
p. 156 p. 157
Occasionally the Bards kept Coelbrens of simple letters, that is to say, one letter on each piece of wood; and by putting them together, according to the requirement of word and name, they mutually communicated one with the other, and understood and acquainted themselves with each other, without speech, and in secrecy. Every one of the pieces of wood was four sided, having six surfaces to each, and a letter on each surface, differently coloured, so that what was wanted might be obtained at sight without much searching. The arrangement of twenty-four was found to be the best for those Coelbrens; and from obtaining mutual knowledge by means of the said Coelbrens secrets were ascertained, which caused much astonishment as to how it was possible.