What self-supporting staircase concept did Leonardo da Vinci draw with two interlocking spiral flights?

Leonardo da Vinci drew a double helix staircase with two interlocking spiral flights.

The best-known example is associated with the Château de Chambord in France, whose central staircase has two spiraling flights that wrap around one another without meeting. People can ascend one flight and descend the other while remaining visually close, yet the routes do not cross. Leonardo is widely credited with designing the concept, although the historical evidence linking him directly to the final staircase is not conclusive.

The staircase is often described as a double helix because its two routes resemble the intertwined strands of a molecular helix. Chambord’s construction began in 1519, several years after Leonardo’s death, so he could not have supervised the completed building. The design is an architectural concept rather than a powered machine, but it demonstrates Leonardo’s interest in movement, geometry, and elegant structural solutions.