The log seat
This swing seat was made using two logs, each about 85mm (3 3/4") in diameter.
The logs were 700mm (28") long and the spacing between the ropes were 500mm (20").
The rope went down between the two logs, wrapped around the two logs once, threaded up between the two logs and tied with three half hitches.
Result: the seat looked esthetically pleasing in the garden but the kids didn't seem to like it all that much as it was definitely lacking in comfort and usability.