- Arranged the known elements in order of RAM (now ordered by atomic number)
- Elements with similar physical & chemical properties were in the same group e.g. metals with low boiling points
- He swapped other elements over if he thought they fitted into another group better based on their properties.
- He left gaps for elements he thought were yet to be discovered
- He made predictions about the properties of elements yet to be discovered, later they were found to be very close to the actual properties