Bohr's way to defining complementarity

• Documental evidence enlightens the genesis of Bohr's presentation of complementarity.
• The real content of Bohr's talk at the Volta Conference in Como in 1927 is disclosed.
• Complementarity was introduced by Bohr only in a very concise way in Como.
• Bohr's interaction with peers possibly substantiated his presentation of complementarity.
• Relevant topics were added to Bohr's papers only after discussing them in Brussels.

We go through Bohr's talk about complementary features of quantum theory at the Volta Conference in September 1927, by collating a manuscript that Bohr wrote in Como with the unpublished stenographic report of his talk. We conclude – also with the help of some unpublished letters – that Bohr gave a very concise speech in September. The formulation of his ideas became fully developed only between the fifth Solvay Conference, in Brussels in October, and early 1928. The unpublished stenographic reports of the Solvay Conference suggest that we reconsider the role that discussions with his colleagues possibly had on Bohr's final presentation of the complementary sides of atomic physics in his 1928 papers.