What do the plans mean by "lay one ply of BID at 45 degrees on the fuselage bottom, then run the second ply the other way"?

Apply the first ply of BID at 45 degrees to the longitudinal axis of the bottom (the long direction). Now, imagine standing at the front edge of the bottom and suppose that the selvage is ascending away from you as you look from left to right. Now, the second ply of BID is also positioned 45 degrees to the longitudinal axis but this time the selvage would be descending towards you when looking from left to right.

This insures that your 1" BID overlaps in the first and second plies do not lie on top of or parallel to each other. Your BID overlaps from the first and second plies will make an "X" pattern if you do things correctly. More.

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