I want to know the answer to the following question:
When was Principles of Phonology published?
All the background I'm provided with is that "One of its leading members was Prince Nikolai Trubetzkoy, whose Grundzüge der Phonologie (Principles of Phonology), published posthumously in 1939, is among the most important works in the field from this period.". Is that enough to answer the question?
Answer: yes