However, single knots can tend to lie quite flat, without much volume. To combat this, the wig factories use split knots. These add a LOT of volume to the hair, but in my opinion they look horrible! Also, I believe that split knots shed much more easily than single knots, which is why factories often resort to using double split knots everywhere except the hairline. This just makes for an ugly end result, as far as I'm concerned, and the reason why a lot of the factory wigs still look 'wiggy' and fake.
Cross-knotting allows the use of single knots, but adds volume to the hair. It also adds a more freestyle direction to the knotting so that it's not set so much in a single direction.
The idea is that every second row of hair is knotted in an alternating direction. The best way to get the idea is to see it illustrated, so here's a diagram showing the very basic concept:
|Cross-knotting / 2-way knotting|
In this example, we want the general direction of the hair coming forward from the crown, towards the hairline. So each row is knotted at slight, alternating angles to the final direction of the hair.
To add even more volume and lift to the hair, a technique called REVERSE cross knotting is used, in which the hair direction is knotted in the exact opposite direction from the final direction you require. Using the same example, again with the final hair direction being forward from the crown towards the hairline, it is actually knotted backwards at alternating angles.
When the hair is finally brushed into its final direction, it retains much more lift at the root, giving a very natural volume to the hair.