[Revised Dec 1, 2008]
This is a way to join two sheets without glue, along a common mountain-fold, at a single point. It uses the ‘dimple’ method I’ve been discussing in recent posts. It's a pretty tight lock; for two free-moving sheets I don’t think I’ve seen one as tight.
1. Valley fold through both sheets, about 2 or 3 cm from the edge to be joined.
2. Remove one sheet, turn it over.
3. Reinsert.
4. Make a diamond dimple through both layers. (Put your hand between the layers from behind, and press in with a finger.) Sharpen the folds, both the diamond mountain fold and the valley at its center.
5. Close the dimple, sharpen the triangles that form next to it. (The far corners of the triangles are arbitrarily located so you may as well align one of them with the cut edge of the overlapping sheet and the other symmetrically).
Step 5 will also make both sheets conical. It is best if the cones of both sheets are oriented in opposite directions--otherwise you will have to invert one of them later.
6. Put a valley fold running through the apex of each of the triangles formed in step 5.
This is how it should look, front and back:
That's it! The lock is formed.
7. The next step is optional. The sheets are not flat; if you like, you can flatten them (from the tip of the cones) with a shallow triangle reaching to the edges of the paper. --For many purposes this step will not be necessary, and it does not add to the tightness of the lock.
By the way, I discovered this locking behavior while exploring what happens when you close a dimple on a single sheet (it locks up a section of the sheet), but it was obvious right away that the move could tie together two sheets too. And more than two as well; but the greater the number of sheets joined at a single point, the less sharp the folds of the exterior sheets and hence the less tight the lock will be.
Of course, because the bond is at a single point, it's more impressive to join two sheets at their corners, rather than at an edge as in the diagrams. The photo up top shows two squares joined at their corners with a Dimple Lock--its strength being tested via the famous 'Sandal Test'...
Saadya
Invented November 15, 2008.