Brassomatic 45 degree petroleum fittings:

Flare fittings make joints which are well adapted to high pressure service and have exceptional resistance to mechanical pull out. They are specially adapted for use where a joint is to be subjected to periodical disassembly as they will give repeated tight reconnection.
Tubing may be either “Single or “Double" flared in making flare fitting joints. When working with similar tubing, the double – flare is recommended to insure against cracking or splitting of the tubing in flaring.