Origami & Folding
what one sheet can do
Concept

Curvature — where it appears

How sharply a curve bends at a point, taken as the reciprocal of the radius of the circle that fits it best there. A curved crease's curvature in the flat sheet is what its rulings are drawn against, and folding cannot change it — everything a curved fold produces is curvature out of the surface instead.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

the crease, its rulings, and the line where they crossan elliptical crease, rulings at 52° from the tangentthe shortest ruling0.0998the tightest radius0.1274their ratio0.7833nothing on the crease pattern marks this line, and no amount of paper moves it

Where the rulings run out

A curved fold's surface is made of straight lines leaving the crease, and the lines are not parallel, so they cross. Past the first crossing there is no surface: two points of the paper have been sent to one point of space. The boundary is a curve nobody drew, no crease pattern shows it, and it sits at the sine of the ruling angle times the crease's own tightest radius — on every curve tried.

material · Curved creases how far each of a curved fold's two surfaces reaches before its rulings crossas a share of the crease's own tightest radius of curvatureone surfacethe othershare boundeda circular crease at 0.40.389—100% / 0%a circular crease at 0.70.644—100% / 0%a circular crease at 10.841—100% / 0%a circular crease at 1.30.964—100% / 0%an elliptical crease at 0.40.389—100% / 0%an elliptical crease at 0.70.644—100% / 0%an elliptical crease at 10.841—100% / 0%an elliptical crease at 1.30.964—100% / 0%a parabolic crease at 0.40.389—100% / 0%a parabolic crease at 0.70.644—100% / 0%a parabolic crease at 10.841—100% / 0%a parabolic crease at 1.30.964—100% / 0%a wave at 0.40.3890.38950% / 50%a wave at 0.70.6440.64450% / 50%a wave at 10.8410.84150% / 50%a wave at 1.30.9640.96450% / 50%a dash is a surface whose rulings never converge, which is a surface with no boundary of this kind at all

Only one side can run out

A curved fold has two surfaces and every reach ever computed here has been one of them. The closed form's denominator is the crease's curvature plus the rate the ruling angle turns at, and crossing to the other surface negates both — so at any point of any crease at most one of the two surfaces can have its rulings converge. A crease that never changes the way it bends therefore has a surface with no such boundary at all, anywhere along it.

material · Curved creases what happens when the ruling angle is not constantthe share of the crease each surface is bounded over, and how far it reaches thereturning rateone surfaceits reachthe otherits reach0.00100%0.29560%—0.25100%0.23500%—0.50100%0.19090%—0.75100%0.14840%—0.90100%0.11990%—1.00100%0.09930%—1.1086%0.077314%2.95101.2580%0.041320%1.17461.4075%0.000025%0.7250the crease is a circular crease and the angle runs 1.4 plus the rate times a sine, so the rate is how fast it turns against how fast the crease bends

An angle that turns faster than the crease

Which of a curved fold's two surfaces runs out is decided by a sum of two rates — how fast the crease bends and how fast the ruling angle turns — and every measurement so far has set the second to zero. Let it turn and it carries the sign on its own: past a rate of exactly one, a crease of unchanging curvature bounds both of its surfaces, which no constant angle on that crease can do. Below that rate the turning costs reach without changing anything else.

material · Curved creases

Named alongside it

The objects these essays reach for when they reach for this one.

Curved creaseDevelopable surfaceEnvelopeIdealisationRuling

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