The rules at that time stated that each crew (double crew carried) must rest twice and one rest period must be at least four hours. For example on a 19 hour flight the command crew would return to their seats one hour before landing. They would have rested the previous 5 hours while the relief crew worked. They would work the 5 hours before that while the relief crew was resting. The command crew would have flown from take off but the time from top of climb to the start of the first 5 hour rest period was divided in two and shared. This usually meant the command crew working for approximately the first 4 hours of the flight, resting about 4 hours, working 5 hours, resting 5 hours and working the final 1 hour. Although it sounds a strange way of doing things I found it excellent as we always arrived feeling relatively fresh and ready for the room party.