The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
p. 154 p. 155
The mysteries of the Bards, that is to say, the secret Coelbrens, are small ebillion, a finger long, having notches *, so that they may be used by two persons or more, who are confidants. It is by placing and joining them together, with reference to what is secret, that words and phrases are formed; and by bundling them into words, according to secrecy, missive epistles and secret books are constructed, the meaning of which no one knows but confidants; nor is it right, according to usage and troth, to divulge the same. They are called the Charms of the Bards, or Bardic Mystery.
Secret Coelvains are similar, made of small stones bearing the marks of mystery; and it is by disposing them, according to the arrangement and art of the Secret, that necessary sciences are demonstrated. And where such Coelbrens exhibit the number of the letters of the Historical Coelbren, let them be made secret by changing one letter for another, so that it be not ascertained except from the necessity and declination of the same letter twice in the same meaning and power.
The Coelvain of History will be, in respect of the historical letters, according to their authorized and fixed order; that is, instead of pillwydd there will be a Peithynen of slate, having on it the reading, inscribed with a steel spike, or a sharp flint.
155:* p. 154 The virtue or charm attributed to letters, as here explained, accounts very clearly for the etymology of rhint, (a rhin,) a notch.