Each token had a different shape and stood for a different kind of goods - grain, animals, etc.

Accountants used the tokens to keep track of who owed what to whom.

The accountants used to seal the tokens in hollow balls of clay to keep a permanent record, but once the tokens were sealed in the ball - or "envelope" - the contents of the envelope were no longer visible.

So, the accountants impressed the tokens into the side of the envelope before sealing them inside, leaving a record of what was inside.

These impressions were the precursor to writing - a two dimensional mark standing for a three-dimensional object.

Eventually, they realized that the marks were enough of a record by themselves, and we are on the road to writing.