This joint uses a butterfly key to hold the two pieces together. The thickness of the key equals to the thickness of the material. I found that butterfly joint is very tight. Once clamp it together, it's hard to take it apart. One problem is that my material has different thicknesses at different area, or because my material is not flat, even though I used nail to fixate it on the board. So some shapes were cut very well, but some shapes were not cut out completely. It took me much time to use a knife to pull the shape out of the wooden material. Next time, I will measure the material for several times, and add 0.001'' to the thickness.
Also because the butterfly key has sharp angle but the pocketed area has round corner, the joint doesn't fit very well. There's a thin gap between them, but it's very tight though.