This is one of the earliest forms of record keeping. The Mesopotamians made rectangular clay squares and used a stylus to indicate how much work was done each day – how much grain was planted or harvested, how many animals were slaughtered, how much water drawn, how many slaves needed. Then each was laid to dry and slipped into a clay envelope and shelved for future reference. Eventually those records on the shelf became plant based scrolls, then books and the clay was used to create shelf markers – hanging tags that described what was in a scroll. An index card, similar to those we once used in the card catalog at the library and the spine stickers that guided us to just the book we were sent to find. A neat bit of history.