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Comment by usrusr | Hacker News Reader

Question: there is a "rule" separating "no cuts" from others, let's call it a purity class. Is "no arbitrary folds" also an established purity class? When I was a kid with too much paper at my disposal, arbitrary folds felt like chatting. Arbitrary folds would be any fold not defined by either a side or existing fold aligning with another side or existing fold or by a corner aligning with another corner. Many intermediate folds would be done just as guidelines for layer folds, and four the structural effect they would have after getting undone. A bit like the "only lines and circles, no measurements" rule in geometry.

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I'm with you on this one.

Virtually all of the instructions on the site begin with folding the paper in half in some way. However, this one breaks that rule:

https://www.foldnfly.com/15.html#Spin-Plane "1. Fold the paper in half about two inches before the edge."

But it doesn't have to! It could just as well have started with a center fold and then folding the corners to the center to measure, then folding that over to measure again. The measurement folds don't have to even be full folds -- you could just fold only at the very edges to lock in the measurement. Also, this plane ultimately has a center fold in it, so it could easily have begun with one.

Going to be tricky since the leading edge of the wing where it meets the nose is often 'arbitrary' in this sense.

Paper airplanes are generally symmetrical, which means you can begin by folding the paper in half and using that fold to determine where the nose is. Unless you mean something else?

I'm talking about the leading edge of the wings, where they meet the fuselage. How far up that is from the tip of the nose is often not based on any existing edge or fold.

The fold that separates the "fuselage" from the wing? Not the leading edge (except in one point), but you are right, that is the one where many otherwise non-arbitrary planes make an exception. My favorite designs, front heavy with a decoratively complex leading edge would usually have plenty of potential guidepoints for the fuselage fold though. Good times, bringing up those memories makes me wonder when exactly I stopped.

Yes, that fold, but specifically that leading point of it. Often the back can have a reference point, like having the edge of the wing meet the bottom rear corner of the fuselage. But yeah, the fold in general is often more arbitrary. But not really; you would try to fold a plane as consistently as possible, so something like 'begin the fold 1/8" from the tip of the nose' isn't arbitrary at all.

Similarly, you often don't want to fold right to a crease, as it can cause bunching. It's often a better idea to leave a mm or so of extra space for inward folds. Doesn't make the design less pure or anything though.

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