Description:

It takes the form of a Right-Angled Triangle oriented with the hypotenuse parallel with HR.
It consists of 3 layers:
The top and bottom layers are each a single layer of paper.
The middle layer consists of 2 right-angle triangles, one each side of the vertical axis of symmetry. Each triangle is a double layer thick.

The hypotenuse are the raw edges of the paper, and the right angle is placed at the centre of the original square.

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

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Method 1:

1. Align a square with the HR, colour side down 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. The paper should be colour side up.
The diagonal creases should now be mountains.

4. Pick the paper up, and fold along one of the diagonal mountain creases. This results in a right-angle triangle 2 layers thick, with its hypotenuse being the folded edge, running left to right and facing the ceiling.
At each end of hypotenuse 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 other diagonal 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 the model 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 right-angle triangle, such that there are 2 layers, each layer consisting of a right-angle triangular flap on either side of the vertical centre line (4 triangular flaps in all).
The right angles of the 4 triangular flaps all touch at the centre vertical axis.

6. The usual way to orientate the Waterbomb Base is with the hypotenuse closest to you and aligned with HR.

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 hypoteneuse 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. Turn the model over from left to right.
Repeat step 4, but this time it will be point C that is on the left.
You will again end up with a triangular flap on top of the paper on the left, with line CD lying along the centre vertical line.

6. You now have a diamond shap comprising 4 layers of paper:
- The top layer is a triangular flap left of the vertical centre line, 2 layers thick.
- The 2nd and 3rd layers are are both diamond shaped.
- The 4th layer is a triangular flap right of the vertical centre line, 2 layers thick.

7. Orientate the diamond shaped model with point A nearest you.
Insert your index finger of one hand between the 2nd and 3rd layers, thereby separating the 2 layers of the diamond shape. Press down on the 3rd and 4th layers of the model at point A, keeping it fixed to the surface of the table. 
With your other hand, grip the 1st and 2nd layers between your thumb and index finger at point A, and pull these 2 layers up and away from you, while the other hand is still holding layers 3 and 4 to the table. 
As you pull, the model will start to form into a 3d diamond-shaped pocket.
As you continute to pull, the model will form into a 3D triangular shaped pocket, with the opening of the pocket facing you.

8. Flatten the model into a 2d right-angle triangle by pressing a hand on each outter side of the pocket. 
There flat right-angle triangle now formed comprises 2 layers. Each layer consists of a right-angle triangular flap on either side of the vertical centre line (4 triangular flaps in all).. 
The right angles of the 4 triangular flaps all touch at the centre vertical axis.

9. The usual way to orientate the Waterbomb Base is with the hypoteneuse closest to you and aligned with HR.

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