The K-20 Moth is my favorite sport plane. It's somewhat tricky to make, but much more fault-tolerant than the K-19.
The folding is held together by a system of tucked-in flaps. The fuselage is not held together at the top by the folding, but holds together in flight fairly well by the lift on the wings.
Under the main wings are some small wings. These look kind of cool but do seem to decrease the performance somewhat. I've made some prototype versions which eliminate these little wings and fly better, but haven't standardized a version of that yet.
The current standard version is the one I've been using for years and is capable of flying very well. I've recently discovered that small wingtip "zigfins" can get it to fly straighter with less inclination to stall. Some exciting reports about those are in the works.
I've made K-20s with various odd rectangular paper sizes from length 18" or so down to about 2 or 3 inches, and the design just about always fits together nicely. I don't think I've tried it with A4 but I'm sure it will adapt easily. The instructions here use 8.5" x 11".
The original version of the K-20 was the K-13B. It's simply the case of a K-20 where in step 2 of the instructions, the corner meets the centerline right at the opposite end instead of in from it. It's an easy way to place the first diagonal folds, and is essentially a short K-20. The "B" in K-13B is for blunt (or semi-blunt) nose. The original K-13 that I first folded didn't have it, but I thought of it almost right away. In the standard K-20, though, I just assume the semi-blunt nose is part of the design and don't bother calling it a K-20B anymore.
I only recently started calling the K-20 the Moth. I'll call the K-13B a Moth as well.
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