Zettel is a collection of fragments which Wittgenstein cut from various of his typescripts and preserved for future use. More than half of the fragments were written in the years 1946-1948, after the completion of Part I and before the composition of Part II of the Philosophical Investigations. This collection may therefore be regarded as a companion volume to the Investigations, adding to both the scope and the Unity of Wittgenstein's chef d'oeuvre.

The fragments were kept in a box and were not strictly ordered. Many have marks showing changes and improvements made after they were cut from the typescripts; some have remarks added in handwriting. Editing the collection for publication was thus a task of considerable difficulty. Since Zettel was first published, further research has been carried out on the fragments and minute comparisons have been made with their typescript and manuscript sources, revealing certain inaccuracies and misinterpretations in the first editing of the work. The second edition corrects these shortcomings and includes new explanatory footnotes; a detailed index, compiled by Stephen Amdur, has also been added.