As the different linguists passed away, their collections of letters and papers were handed over to the Royal Library in Copenhagen. A great portion of the letters, notes and papers of Jens Holt ended up in Aarhus University's archive of University History, however.
The consequence of these filings is that we till this day can follow what many linguists wrote about and discussed between each other. Even though a part of the correspondances contain trivial affairs, there is still a large amount of letters, where the discussions about the foundations of and perspectives on language are fierce and intelligent.
Furthermore, we see how the Danish postal service at the time functioned with an almost superhuman precision, due to the fact that letter-corresponding was the preferred method of efficient communication. Back then, it was quite expensive (and inappropriate) to have long, cross-country conversations on the telephone. Even though the scholars lived fairly close to each other, they still communicated by letters. These letters were carefully kept by the receivers and most of the times, they kept a copy of their own letters; also called a "carbon-copy". Most of the letters we have are typewritten and original, but in quite a few cases we only have a carbon-copy. This makes it difficult to see possible handwritten corrections and notes which were added at the last minute before sending.