Thinskin is designed as a full polyurethane (PU) “skin” base-which gives the impression of hair growing directly out of the scalp. This base has three methods of hair attachment (ventilation): Knotted, V-Loop and Injected. Knotted is created similar to lace systems, but the knots are invisible since they lay beneath the skin. V-Loop is constructed with longer strands of hair woven widthwise across the unit, so hair lays closer to the head. Injected has not knots as hair is heat-injected into the base. As a result, this base is thicker to allow hair to be inserted deeper for durability. This base is $25 extra.