The cuneiform writing, whether derived from the earlier hieroglyphs or developed independently by the Accadians, was employed with all but unlimited fertility by the Assyro-Babylonian civilization. The writing was distinguished from the hieroglyphic in that it was made up, in its entirety, of a single, wedge-shaped or arrow-headed-like character, formed with a metal _stylus_ having a triangular end. By pressing this stylus in the plastic clay of the prepared tablet or cylinder a sharply defined and angular shaped indentation was impressed and, afterward, the clay with its writing was hardened by exposure to the sun or baked by fire into an almost imperishable "record." The all but indestructible character of this material accounts for the large proportion of the Assyrian literature which has been preserved through tens of centuries. Professor Albert T. Clay describes the preparation and use of this material as follows: "The well-kneeded clay, which had been washed to free it from grit and sand, while in a plastic condition was shaped into the form and size desired.... The stylus, which was made of metal or wood, was a very simple affair. In the early periods it was triangular and in the later quadrangular.... By pressing a corner of it into the soft clay, the impression made will be that of a wedge; hence the term cuneiform (from the Latin _cuneus_) writing."[47] The single simple character ( ► ) from which the cuneiform writing was entirely constructed was used in multitudinous combinations and in various positions (somewhat as the Chinese ideographic characters are still used) to record the thoughts and deeds of the primitive Accadians. Great libraries, written in cuneiform, were accumulated in different centers of population; these were transmitted to the succeeding Assyrians and Babylonians. The cuneiform writing was read in the prevailing direction which the characters pointed. The "key" to the decipherment of the cuneiform writing--as that employed in the decipherment of the Egyptian hieroglyphs--was a "lucky guess" by Dr. Grotefend, a German scholar. Following the clue of a few known names on the monuments, verifying by these the conjectural values of six cuneiform combinations, he reached basal conclusions from which, finally, the Assyro-Babylonian scholars have been enabled to read these ancient cuneiform texts and inscriptions with as much assurance as the pages of the Old Testament Hebrew; and so he opened up to view a vast body of the otherwise un-read records of the past. Thus the writings of the great libraries written in this character, as at Assur, Calah, and Nineveh, though buried from sight for multiplied centuries, are now accessible through the labors of the Assyriologists.