The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
Here is shewn the mode of making the Coelbren of the Bards. Small pieces of wood are taken, split, about a cubit in length, and squared, until they are four sided, the width of each side being less than half an inch, that is, its width will be about the length of a barley corn. On these sides the letters are cut; but before they are cut, it is requisite that the edges should be rasped, in order that, when the letters are cut on one side, their heads may not appear as characters on the other sides, except in the rasping.