The received text of the Tao Te Ching is divided into eighty-one chapters: thirty-seven in the Dao section and forty-four in the De. Because eighty-one is nine squared, and nine carries considerable weight in Chinese numerical thought, the division is often presented as significant in itself. The manuscript evidence indicates that it is not.
The Mawangdui silk manuscripts, recovered from a tomb sealed in 168 BCE, are the oldest substantially complete copies. They reverse the order of the two halves, placing the De section first, so that the work would properly be called the De Dao Jing. One of the two copies has no chapter divisions at all; where divisions appear they are unnumbered and untitled.
The Guodian bamboo slips, from a tomb of around 300 BCE, are older still and contain only a partial selection of the material, in a different arrangement, amounting to roughly a third of the received text.
The eighty-one chapter division is associated with the Heshang Gong recension and is standard by the Han period. It is therefore an editorial arrangement imposed on a text that circulated for centuries without it, and the nine-by-nine reading is a rationalisation of the division rather than its cause.
This matters for how the book is read. Chapter boundaries determine where an argument is taken to begin and end, and several of the received chapters combine material that the earlier manuscripts do not present together. Treating the eighty-one as authorial, as evidence that Laozi, whose historical existence is itself uncertain, structured the work numerologically, attributes to an author a decision made by later editors.