Twinthread

[oddball61]

Can somebody please explain how this is a twinthread screw because I only see one thread?

 

[foxhole]

If you take a fine felt tip pen and place in a groove then rotate screw the pen will mark the groove up the screw but only alternate threads as there are two sets of thread .
The photo would look like a single thread until you add colour to show there are two intertwined threads .

 

[oddball61]

Just done that and your right,thanks for the explanation.