The square module (90 degrees)

Introduction -- Making the Modules -- Making the Polyhedra -- Jim's Origami Page
Start with a 4x3 piece of paper, and fold like an accordian into three segments:

Fold the lower left and upper right corners in. Use only the top layers. Unfold:

Valley fold along the dotted line:

Fold the entire piece as indicated

Unfold back to a rectangle.

Remove the excess paper above and to the right of the thick lines. Note that these lines do not fall on any crease lines. This can be done with inside folds or with a pair of scissors if you're willing to do so.

The final piece: