They were wooden plates with their inner surface slightly curved and covered with black wax. A special writing instrument – stylus (with a pointed end on the one side for writing the message and a flat end on the other for erasing it) was used to engrave the message on the waxed surface with normal or cryptographic letters (i.e. with the use of dots instead of vowels, mirror writing, etc.), or, in special cases, beneath the waxed surface (i.e. the message of the exiled Spartian Demaratus from the Persian palace: “Xerxes invades Greece”).
