“Back” is the color of the paper's reverse. You see it wherever a cut piece has been flipped over an edge and lies face down beside the sheet. A new design starts with the back the same black as the front, as if the sheet were one color through and through.
What it does
Every flipped piece is filled with the back color. When a shape cuts the sheet and you choose a direction, the piece turns over about the edge of its own hole, showing the back color by the time it has cleared the hole, and then travels out across the sheet's edge to land beyond it, mirrored. As it lifts, the light catches it and a shadow falls under it; flat again, it shows the plain back color.
Out beyond the sheet you are drawing where paper lands. A shape drawn mostly outside the sheet is mirrored onto the sheet, across the fold of the piece it covers most or else across the nearest edge, and is cut or put back from there.
Paper you put back into a hole does not show the back. It returns to the sheet face up, in the front color.
A hole in the sheet always shows the ground, never a piece. In the artwork export the pieces are drawn first, as a single shape in the back color (joined, so no hairline shows where two pieces meet), and a sheet-sized patch of ground is laid over them before the sheet itself. A piece that strays back over the sheet cannot show through a hole.
In the harmony order, Front, Back, Ground, the back comes second. The Back card has its swatch, name, hex code, padlock and sliders, like the others.
Why it works this way
A flipped piece shows its back because that is what paper does. Turn a piece over and you see the side that was face down. Giving the back its own color lets you imitate every paper there is: papers the same color through, papers white on the back, papers made in two colors.
The starting back matches the front for a reason. The classic notan exercise is cut from one sheet of black paper, black on both sides, laid on white. With front and back the same, each flipped piece reads as part of the same figure, joined to the sheet along its fold, growing out of it like a reflection. Change the back and the pieces separate from the sheet into a second voice.
The metaphor
The back is the verso, the reverse of a leaf, and the gesture is turning it over. The folder of origami knows this well: origami paper is often colored on one side and white on the other, so every fold announces which side is up, and double-sided papers in two colors make the folds themselves the design.
The app's flip is a reflection: the piece is mirrored across the edge it turns over, the way a sheet folds flat. The metaphor holds for the geometry, since a folded piece really does land as the mirror image of its hole. It breaks in two places. First, a real fold keeps the piece attached at a hinge, and a real cut frees it; the app's pieces are cut free and then placed as if folded, so they sit exactly where a fold would put them without any paper joining them. Second, a real back is rarely just a color: it can differ in texture, and thin paper lets printing on one side show through the other. The app's back is perfectly opaque and perfectly flat.
A history
In art
The expanded square, as it is often called in art classes, is this app's exercise done by hand: cut shapes from the edges of a dark square and fold or flip each one outward onto a white ground, so that every positive shape is paired with its negative. It is a common classroom exercise in design and notan, and its origin is hard to trace.
The back was a subject at the Bauhaus. In Josef Albers's preliminary course of the late 1920s, students folded, scored and cut single sheets of paper to find out what the material could do, and a folded sheet always shows both its sides. The word kirigami, for folded and cut paper, reached English through Florence Temko's 1962 book of that name. She built it from Japanese kiri, cut, and kami, paper, though it turned out the word already existed in Japanese.
In printing
Printers call printing the second side of a sheet backing up. On most presses the sheet goes through once, dries, and is turned over for a second pass; a perfecting press prints both sides in a single pass. How the sheet is turned is part of planning the job: in a work-and-turn, the same plate prints both sides of a sheet, which is then cut in two to give two finished copies. The back also matters as a problem. Show-through, where printing on one side is visible from the other, is why fine books are printed on opaque papers.
Some papers are made with two different sides on purpose. Paper coated on one side only, the trade's C1S, has a smooth, bright face for printing and a plainer back, for labels, covers and cards. And every relief print is itself a flip: the block is carved in mirror image, and pressing it face down onto paper turns it the right way around.
In technology
Three-dimensional graphics has to decide, for every surface, which side you are looking at. Each triangle in a 3D model has a front and a back, told apart by the order of its corners, and renderers commonly skip back faces entirely, a saving called back-face culling, because on a closed object they are hidden anyway. A surface that must show both sides, such as a leaf or a sheet of paper, is marked two-sided and given a material for each.
Web pages have the same idea. The CSS property backface-visibility decides whether an element turned away from the viewer shows its reverse, which is how card-flip effects are made. Studio Notan's flip animation is simpler and flatter: it squeezes the piece toward its fold line and out again, like a coin seen edge-on, and once the piece has turned past edge-on it is drawn in the back color.
In professional software
In 3D software the back is the back face: every surface has a front and a reverse, and the program must be told what to show on each. In 2D and cutting software the back usually appears as a mirror image.
- Adobe Illustrator. The Reflect tool (
O) flips a shape across an axis. ⌥-click (Alt-click on Windows) sets the point the axis passes through and opens a dialog with a ‘Copy’ button, which leaves the original in place. - Blender. The ‘Face Orientation’ overlay shows back faces in red. The ‘Geometry’ node’s ‘Backfacing’ output, 1 on the back of a face and 0 on the front, can mix two materials, one for each side.
- Autodesk Maya and 3ds Max. In Maya, each shape node has a ‘Double Sided’ switch under ‘Render Stats’; untick it and the back of a surface does not render. For two different sides, Arnold, the renderer that comes with both programs, has a Two Sided shader (‘aiTwoSided’ in Maya) that takes one material for the front and another for the back.
- Pepakura Designer. It unfolds a 3D model into flat parts to print, cut and fold. The printed side becomes the outside of the model, and each fold is marked as a mountain or a valley, according to which way the paper turns.
- Cricut Design Space. Iron-on vinyl is cut from the back, so the ‘Mirror’ switch on its mat reverses the cut. The design reads correctly once it is turned over and pressed.
What carries over. A surface has two sides, and the side facing away still exists, so decide what it shows. In 3D, a surface that renders dark or disappears often has its normals facing the wrong way; the fix is to flip them, not to repaint. In cutting, anything cut from the reverse of a material must be mirrored first, just as a notan’s pieces land mirrored across their folds.
Try this
- Cut and flip a shape on the starting black sheet: the piece joins the sheet along its fold. Now change the back to white and watch the piece separate.
- Set the back to the same color as the ground. The pieces vanish, and only the holes in the sheet remain.
- Use the replay button to watch every flip turn over again and see the moment each piece changes to the back color.