Where a seam has to be glassed and it is difficult to make the glass sit flat because of a complex curve then a double bias tape is used.
It is also useful for complex 3D curves like the tips of centreboards and rudders.
Commercial double bias tapes can be bought but you can make your own for smaller areas.
Double bias tape is also useful in high strength applications as all the threads cross the join between the two pieces being taped. Not essential for most craft but occasionally there is a high strength application
Cutting the glass for the tip of a centerboard or other curved area.
The tip is glassed with a piece of double bias cloth first. Double bias might sound a bit technical, but all it means is cutting out a piece to do the tip from the glass cloth you have bought, but instead of following the edges of the cloth the rectangular piece needs to be cut out at 45 degrees to the edges.
This means the threads in the piece will be at 45 degrees to the edges of the piece that you have cut out. It doesn’t need to be exactly 45 degrees if that won’t come out of the offcut from the cutting of the cloth for the body of the foils in the previous step.
This is by far the BEST way to cut out glass when it will have to curve two directions at once like on the rounded tips of the foils.
One of the weird things about double bias is that if you pull one end the tape gets longer – but a lot thinner – and vice versa – it is quite difficult to keep in shape so it is important to handle it carefully and cut out a piece bigger than you really need. Probably twice as wide and about 30 percent longer than you think you need.