How to Double Open Sink the Sides of a Folded Base
To double open sink the sides of an origami base, open the indicated layers, sink the larger marked region inward, then reverse the smaller inner region outward. Reclose the base along its existing creases and repeat on the other indicated side. The two reversals create a nested change of direction in the paper. The Yoshizawa–Randlett system reference describes an open double sink as an open sink followed by another in the opposite direction.
This guide is for a folder working through a diagram that calls for double open sinks along the sides of an already folded base. Keep that diagram beside you: the model determines which layers move, where the two sink boundaries lie, and whether the operation repeats on the left, right, front, or back.
What Does “Double Open Sink” Mean?
An open sink turns a region inward while allowing the affected paper to open flat during the maneuver. Robert J. Lang identifies that ability to flatten the region as the defining feature of an open sink. His explanation also shows why an apparently similar closed sink can produce a different arrangement of hidden layers. See Origami Diagramming Conventions.
The word double describes the two successive, opposing sinks within the affected region. Repeating the completed operation on the opposite side is a separate instruction. Taro’s Origami Studio lists double open sink and double closed sink as distinct techniques in its 1st-Kyu Red Belt course.
A useful way to follow the paper is inward at the outer boundary, outward at the inner boundary. This describes the motion relative to the surrounding paper; it does not prescribe mountain and valley symbols for every viewing angle. Follow the crease directions shown in your particular diagram.
What Should You Check Before Opening the Base?
First, match your base to the drawing immediately before the sink. Check the orientation, the number of visible flaps, and which edges are open. Then look ahead to the result drawing. Identify which outline changes and which flap must remain accessible.
Use this preparation checklist:
- Locate both sink boundaries on the side you will work on.
- Identify the layers included in the operation.
- Find any preceding fold-and-unfold instructions that establish those boundaries.
- Check whether the repetition applies to the opposite side or to layers behind.
- Keep track of the base’s centerline so you can restore its orientation.
These checks matter because a short sink instruction can depend on several preceding steps. In Francesco Massimo’s original Dragon / Dragón 3.0 diagrams, page 4 establishes a bird-like arrangement before step 30 calls for a double open sink. Step 31 then specifies a separate open sink using earlier creases and explicitly says to repeat behind. Read each instruction within its own sequence.
How Do You Prepare the Two Sink Boundaries?
Make the prescribed precreases before attempting the collapse. Fold accurately to the diagram’s reference edges or points, crease, and unfold. If the instructions establish the same crease from the reverse side, complete that preparation too.
Origami.me’s open-sink tutorial demonstrates this principle on a square base: crease the intended boundary, turn the model over, reinforce it in the opposite direction, then open the paper to perform the sink. Its example concerns a single sink, but it provides the preparation method needed before adding the second reversal.
For your double sink, distinguish the outer boundary, enclosing the larger region, from the inner boundary, enclosing the smaller region. Those labels describe their relationship when the affected paper is opened. On a flattened base, the same boundaries may appear as short lines across a narrow side section.
Do not choose new distances simply because the instruction gives no measurements. Use the diagram’s alignment references. If a boundary disappears under another layer, open enough paper to trace its continuation before proceeding.
How Do You Make the First Inward Sink?
Work on one indicated side first. The following sequence adapts the open-and-recollapse method shown in Origami.me’s beginner’s guide to the side region identified by your model’s diagram.
- Separate the participating layers. Open the side pocket or flap arrangement until you can see the outer sink boundary. Release adjacent folds temporarily if they prevent access.
- Support the boundary. Hold the surrounding paper near the crease rather than compressing the whole base.
- Reverse the larger region inward. Guide the paper across the outer boundary, following the prepared creases around the affected region.
- Start restoring the base. Bring the surrounding layers toward their original positions while leaving access to the smaller region needed for the second sink.
Pause before pressing the base completely flat. Compare the first reversal with the diagram’s intended direction. If you have merely folded an outside flap over the surface, reopen it and trace where the paper should enter the base.
How Do You Add the Second, Outward Sink?
Keep the first sink in place and open the smaller region inside it. Reverse that smaller region in the opposite direction along the inner boundary. This is the second part of the double sink described in the Yoshizawa–Randlett system reference.
Guide the inner reversal while supporting the strip of paper between the two boundaries. Preserve that strip’s folded shape as the smaller region moves outward. Avoid pulling the whole first sink back to its starting position.
Reclose the base gradually, bringing its original structural folds together around the completed double sink. Before sharpening the folds, inspect the exposed edges and compare the result with the next drawing. The useful check is whether the two reversals remain present after the base closes.
Now repeat on the other indicated side. Reorient the model to match the diagram again before starting. For a mirrored operation, use the corresponding reference creases rather than estimating the second side from the first side’s visible width.
How Can You Diagnose a Sink That Will Not Flatten?
Use the following checks as a practical diagnostic sequence derived from the open-sink method. Reopen the affected region before correcting it so you can see which crease or layer is responsible.
| What you observe | What to inspect | What to do next |
|---|---|---|
| A bulge remains between the two boundaries | The inner reversal may be incomplete | Reopen the smaller region and follow its boundary through all participating layers |
| The first sink disappears during the second | The outer boundary may have unfolded | Restore the outer inward sink while supporting the strip between boundaries |
| A flap needed in the next step is trapped | The layer arrangement may differ from the diagram | Compare hidden layers and release any flap included accidentally |
| The two sides have different outlines | Their precreases or layer selections may differ | Open each side enough to compare corresponding boundaries |
| A new diagonal buckle appears | A section may be collapsing away from its prepared crease | Ease the region open and return it to the intended crease |
A trapped flap deserves particular attention. Lang explains that open and closed sinks can share visible crease locations while differing in hidden folds and layer distribution. Matching the outer silhouette alone therefore cannot confirm the operation. Check the inside arrangement against the instructions in Origami Diagramming Conventions.
How Do You Know the Side Sinks Are Finished?
Before continuing, return the base to the orientation shown in the result drawing. Check the outline, both sink boundaries, and the availability of the next working flap. Where the design calls for matching sides, compare their corresponding edges before pressing the creases firmly.
The most useful finishing test is to trace the paper through the worked region: it enters at the outer boundary and reverses outward at the inner one. Once that nested structure matches the diagram and the base closes into the required shape, sharpen the creases and continue with the model.