The accepted industry standard for diameters is +- .0005”, and in 5X material these day most manufacturers are at .0065”, which technically if you rounded off would be 4X, but since it has become standard across the board, Orvis elected to go this route as well. If they chose a diameter of .0055”, it would technically be 5X but when compared in the marketplace to other 5X materials, Orvis' leaders would be slightly finer in diameter and thus would look weaker than they actually are. **They used the improved clinch knot, probably the most common knot used in fly fishing and thus a real-world test. The knots were all tied by the same experienced knot tier and soaked overnight. This test is a lot more valid than simple dry break strength.