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The nature of feather construction
The arrangement of barbs, barbules, and barbicels is important to understand when marrying feather strips for a wing on a wet fly or Atlantic Salmon fly. The marriage of a strip of upper barbs to a correctly matching strip of lower barbs is quite easy if the face side of the upper strip is placed slightly behind the face side of the lower strip. This allows the hooklets on the barbules of the top barb of the lower strip to have opportunity to grasp the grooved edge (dorsal flange) of the barbules on the upper strips' bottom barb.
If the strips are overlapped immediately above and below one another, or perhaps the upper strip is in front of the lower strip, due to their arrangement on the barbules, a complete interlocking of hooklets to flanges will not occur. If a strip is overlaid with another strip, but the matching strip is upside down, this arrangement of barbicels will not allow the strips to marry. If a right strip is overlaid with a left strip, even though the proximal to distal arrangement of the barbules is correct, no reliable marriage will occur, because the hooklets and flanges do not align.
Many feathers develop fault bars across the vane. As feathers grow, a disruption in cell development may occur leaving distinct lines across the vane generally perpendicular to the shaft. These are due to stress, other abnormal conditions, or may be present under normal conditions. A fault bar's appearance is due to underdevelopment of barbules or total lack of barbules in the area of the disruption.