Triangle (60 degrees)

Probably the most important variation to learn because triangles exist in most polyhedra. Also, many other variations fold similarly.

Prerequisite: A completed base Penultimate unit.

Step 1. Using the top flap only, fold the top edge of the little triangle to the crease sitting at 45 degrees.

Top flap's little triangle folded so its top edge meets the crease sitting at 45 degrees

Step 2. Fold the newly formed crease in Step 1 to the crease sitting at 45 degrees. Keep the fold in place.

Crease from Step 1 folded again to meet the 45 degree crease, with the fold held in place

Step 3. Fold the newly formed crease in Step 2 to the long diagonal crease in the middle of the piece.

Crease from Step 2 folded to align with the long diagonal crease running through the middle of the piece

Step 4. You now have a 60 degree angle. Congrats! The issue is that the flap is currently too big for insertion. You can either cut along the red lines drawn in the picture, or fold along these lines and tuck the extra paper using inside folds—shown in Steps 4a-4c.

Completed 60 degree angle with red lines marking where to cut, or alternatively fold and tuck, the oversized flap

Step 4a. Tuck the top right triangle inside, as shown. Depending on which side of the strip you initially chose, there may be 1 or 2 triangles to tuck.

Top right triangle flap tucked inside the unit, with 1 or 2 triangles to tuck depending on the strip side chosen

Step 4b. Using only the top flap, fold the bottom right corner towards the foot of the vertical crease. Repeat on the back side for the remaining flaps.

Top flap's bottom right corner folded toward the foot of the vertical crease, to be repeated on the back for remaining flaps

Step 4c. Tuck each of the small triangles created in Step 4b inside, as shown. There may be 2 or 3 of these triangles.

Small triangles from Step 4b tucked inside the unit, completing the finished triangle module