Peierls visited Bohr in Copenhagen repeatedly for conferences or just for consultation. The first visit was probably in 1931. The letters are about physics, about the troubles that L.D. Landau was having, and about arrangements for mutual visits.

About 1938 Bohr, Peierls and Placzek (who then spent much of his time in Copenhagen) discussed nuclear reactions and reached conclusions which were to be embodied in a joint paper. Drafts of portions of this paper were discussed in Copenhagen, on visits by Placzek to Birmingham, and by letter.

No satisfactory version had been arrived of when the war interrupted communications. Various drafts had, however, been shown or given to many other physicists so that the 'paper' was frequently cited - probably the most frequently cited unpublished, indeed unwritten, paper. After the war, there were further attempts to agree on a text, but they were finally abandoned.

Correspondence after 1946 (MS. Eng. misc. b. 203/C.32-MS. Eng. misc. b. 203/C.36) is addressed to, and signed by, 'Uncle Nick'. This refers to Bohr's nom de guerre Nicholas Baker, under which he travelled and worked during the Second World War. His older colleagues and friends adopted the name permanently in their subsequent dealings with him. Bohr's son, Aage, who escaped to England with him in the war, was known as James Baker, or Jim.