I took a couple of shots to illustrate why I believe that good ventilation can mean the difference between an acceptable piece and an exceptional piece.
I began work on the second hairpiece, and I am being even more conscious of the direction in which the hair is vented. I am also, once again, ventilating the entire piece using single strands. The section shown here is double-knotted, but as I progress to the top and front of the piece I will begin single knots.
Some wigmakers believe that the knots and ventilation direction are not critical at the back of a piece, and that it's only once you get to the hairline that you need to be concerned about those aspects. While I agree that this is often true in a full, long wig, or one that will be styled to hide the knots in the back, the importance of good ventilation becomes much more obvious in a piece with lower density, and one that will be cut in a short style, such as a man's hairpiece.
As you can see here, the direction that the hair is vented in allows it to conform naturally to the shape of the head. If this had all just been ventilated back, as is often done, the result would have been far less natural. In the case of a hairpiece, especially, where the need to blend with existing natural hair is the goal, these considerations are critical.
I also believe that single strand knotting allows the hair to move and flow much more realistically, while keeping the knots small and undetectable.