Cuneiform is a system of writing developed by the Sumerians near the end of the fourth, and during the third millennium BC The word "cuneiform" is derived from the Latin word "cuneus" meaning "wedge." The pictograms, or series of pictures representing words, were written on wet clay tablets with a sharpened tool, or stylus.

The earliest texts were records of business transactions and administration involving things such as grain, and the distribution of animals. The clay was readily available from the river valleys of the Tigris and Euphrates Rivers. After the scribe, or dubsar, worked on the tablet of moist clay, it would be left out in the sun to dry. The longer it remained in the sun, the harder the tablet became.

The first envelopes were also an invention of the Sumerians. These were also made of clay. Basically, after the tablet was hard enough, it was encased in a thin layer of clay. Whatever was written on the tablet, was written on the front and back of the outer clay shell. The Sumerian scribe also may have used a cylinder seal to sign his product on all eight sides of the envelope. As far as writing is concerned, the early texts are not written in neat lines with every sign in the appropriate order. The later texts do occur in some sort of order with all the signs for each sentence grouped together in a box. The correct order in which the signs are read is a matter of interpretation.

Source: (Walker 1990 pp. 7, 26-28)