Mereology: A Philosophical Introduction

We could claim instead that the piece of wood is a proper part of the table, but that the table is not a proper part of the wood.

The reason to distinguish between the table and the piece of wood (and to avoid taking the first way out) is presumably that the table is a richer entity, one that instantiates some further properties (for example, some stylistic features—it is in the Art Nouveau style) that the piece of wood lacks.

Once their spatiotemporal coincidence is not taken as decisive evidence that they are identical, it is not clear either whether it serves as evidence that there is a mereological relation between them.