Square (90 degrees)

The square unit is much simpler than the triangle unit. It shows up occasionally, but it is useful to know.

Prerequisite: A completed base Penultimate unit.

Step 1. Fold the top half along the existing main diagonal crease and keep it in place. Then, take the top right corner and fold it down such that the top edge meets the temporary edge in the middle.

Top half folded along the main diagonal crease, then the top right corner folded down to meet the temporary middle edge

Step 2. Reverse the folds created in Step 1. You now have a 90 degree angle! The issue is that the flap is too big for insertion. You can either cut along the red lines as shown, or follow Steps 2a-2e to do inside folds.

Completed 90 degree angle after reversing the Step 1 folds, with red lines marking where to cut or alternatively do inside folds

Step 2a. Fold the entire top right corner down (all flaps) such that the top edge meets the crease sitting at 45 degrees.

Entire top right corner, all flaps together, folded down so the top edge meets the crease sitting at 45 degrees

Step 2b. Tuck the newly created triangle inside. Depending on which side of the piece you originally chose, there may be 1 or 2 flaps to tuck separately.

Newly created triangle tucked inside the unit, with 1 or 2 flaps to tuck separately depending on the side chosen

Step 2c. Using the top flap only, fold the right edge towards the middle as much as possible. You should notice that a point in the middle is blocking you from going further. Keep the fold in place.

Top flap's right edge folded toward the middle as far as possible, stopped by a blocking point in the center, with the fold held in place

Step 2d. Using the same top flap, fold the left edge back to meet the newly created crease in Step 2c. Keep the fold in place.

Top flap's left edge folded back to meet the crease created in Step 2c, with the fold held in place

Step 2e. Reverse the fold made in Step 2c ONLY (not Step 2d). Turn the paper around and repeat Steps 2c-2e for all remaining flaps at the back.

Fold from Step 2c reversed while the Step 2d fold stays in place, ready to repeat Steps 2c through 2e on the remaining back flaps