In Danish, the pair /ø/ and /ø:/ are distinguished from the pair /œ/ and /œ:/. In Swedish, the phonemes /ø:/ and /œ/ are treated as a short-long pair.
In Danish, the pair /ɔ/ and /ɔ:/ are distinguished from /o/ and /o:/. In Swedish, /o:/ and /ɔ/ are treated as a short-long pair.
How did Danish develop a length distinction for the qualities /o/, /ɔ/, /ø/, and /œ/?