Description:

This has the shape of a diamond, with 4 equal sides. It is 1/4 the size of the original square of paper.
It consists of 3 layers:
The top and bottom layers are each a single layer of paper.
The middle layer consists of 2 triangles, one each side of the vertical axis of symmetry. Each triangle is a double layer thick.

The preliminary base is normally oriented in a diamond shape with singular point (the centre of the original square) at the top, and open end at the bottom where you can feel 4 points points: one from the top and bottom layers of paper, and a point from each of the triangles of the middle layer.

It is worth noting that the Preliminary Base is the Waterbomb Base turned inside out, and vice-versa.



Method 1:

1. Align a square with the HR, colour side up and valley fold and unfold
each diagonal in turn.

2. Turn the paper over and valley fold and unfold on the horizontal, and then the vertical centre lines.

3. Turn the paper over so that it is colour side up, and the horizontal and vertical creases are now mountains.

4. Pick the paper up, and fold along the horizontal mountain crease in the centre of the paper. This results in a rectangle 2 layers thick.
At each end of horizontal fold just formed, grip the two layers of paper between your index finger and thumb.
Hold the model up in the air so that the folded edge is horizontal, and facing the ceiling.
Move your hands inwards and downwards towards each other (rotating movement about the centre point).
As you do this, the vertical mountain creases will start to cause the paper to fold along them, causing the centre of the paper to peak into a point. 
Keep bringing your hands towards each other until it has formed into 4 triangular flaps: 2 that you are holding and whose flat faces are facing you; and two flaps perpendicular to these.
If you were looking down from the ceiling onto the central peak of the model, you would see a cross shape.

5. Flatten the whole thing into a 2d diamond, such that there are 2 layers, each layer consisting of a triangular flap on either side of the vertical centre line (4 triangular flaps in all)..

6. The usual way to orientate the Preliminary Base is as a diamond, with the raw edges at 45 degrees to HR, i.e. the point formed at the centre of the original square at the top, and the open part of the base at the bottom.

Method 2:

1. Align a square with the HR, colour side down and valley fold and unfold
one of the diagonals.

2. Valley fold the second diagonal and leave it folded.

3. You now have a RAT, comprising 2 layers of paper with the colour side on the outside.
Align its hypotenuse with HR. The following notation will be used to label it:
- The right-angle at the top is point A.
- The left-hand end of the hypotenuse is point B.
- The right-hand end of the hypotenuse is point C
- The middle point of the hypotenuse is point D.

4. Taking hold of point B, both layers, swing it up to touch point A, so that the fold line BD moves from being horizontal to lie along the vertical centre line AD.
You have now formed a triangular flap on top of the paper on the left.

5. Repeat step 4, with point C being swung to point A, such that line CD lies vertically along line AD.
You will again end up with a triangular flap on top of the paper on the right, with line CD lying along the centre vertical line and touching line BD.

6. You now have a diamond shape comprising 3 layers of paper:
- The top layer consists of 2 triangular flaps 2 layers thick, one left and one right of the vertical centre line.
- The 2nd and 3rd layers are are both diamond shaped.

7. Orientate the diamond shaped model with point A at the top furthest from you.
You are now going to insert the righthand triangular flap (and then the lefthand triangular flap) in between the 2nd and 3rd layers of the diamond, such that the two triangular flaps will be sandwiched between the two outer single layers of diamond shape. 
Do this to the right-hand triangular flap by first unfolding it so that line CD is once again horizontal.
Now, grip all layers of the model at point a with your index finger and thumb of your left hand.
With the right hand, insert a finger between the two layers of paper at point C, thereby opening the diagonal line AC into its two layers forming a triangular pocket. 
With the index finger of the right hand still inserted between the two layers of paper along line CD, and your thumb of the right hand on the outside of the paper along line CD, and the lefthand still holding together the layers of paper at point A, push your right-hand up to make point C touch point A. 
As you do this, 2 new triangular flaps will form, in a plane perpendicular to the original model. One flap will be sticking out toward you, and the other sticking out away from you on the back of the model.
At this point, if you turned your head to look 90 degrees on to the end of the model, you would see a diamond shape, consisting of two triangular flaps about a vertical central line of symmetry.

8. Flatten the model into a 2D diamond, by bringing together the 2 new triangular flaps created in step 7.

9. You now have a 2d diamond, consisting of 4 layers:
- The top layer is a triangular flap on the left of the model.
- The 2nd layer is a single layer of diamond shape.
- The 3rd layer is a triangular flap on the right of the model.
- The 4th layer is the other single thickness diamond shape.

10. Repeat steps 7 and 8 with the left-hand triangular flap, to form a mirror image.

11. You now have a 2d diamond, consisting of 3 layers:
- The top and bottom layers are each a single layer of diamond shape.
- The middle layer consists of the 2 triangular flaps, one each side of the centre vertical line of symmetry.

12. The usual way to orientate the Preliminary Base is as a diamond, with the raw edges at 45 degrees to HR, i.e. the point formed at the centre of the original square at the top, and the open part of the base at the bottom.

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