Here is a comparison of a double threaded insulator on the left, with a normal (single) threaded insulator on the right. On a typical insulator there is a single spiral of 4 threads per inch. On the double-threaded insulator, there are two independent threads going up, each starting on opposite sides, and they wind around each other. There are only 2 threads per inch on each one, giving a steeper angle to the threads. This is highlighted more in the [id=119267217; following picture].