Preliminary base (square base)

The preliminary base, also called the square base, is a traditional starting shape in which a square collapses into a smaller square with four flaps and all the open corners at one end.

With the colored side up, crease both diagonals; turn the paper over and crease both midlines. Now hold the sheet so a corner points toward you and bring the other three corners down to meet it, letting the paper fold along the existing creases. It flattens into a square one quarter of the original area, open at the bottom.

If the paper resists or wants to form a triangle instead, the diagonals and midlines are probably on the same side; they must be on opposite sides. Sharpen all creases before collapsing. The preliminary base leads straight to the bird base and is the first milestone in the Paper Crane.

How to fold a preliminary base, step by step

1
With the colored side up, fold the square in half along one diagonal and unfold, then along the other diagonal and unfold.
2
Turn the paper over so the white side faces up.
3
Fold it in half edge to edge and unfold, then in half the other way and unfold.
4
Hold the paper with a corner pointing toward you.
5
Bring the left, right and top corners down to meet the bottom corner; the sheet folds itself along the existing creases.
6
Flatten it into a small square with two flaps on each side and all the open corners at one end.

Common mistakes and how to fix them

The paper wants to become a triangle
The diagonals and midlines are bending the same way. They must bend opposite ways, which is why you turn the paper over between the two sets of creases.
The collapse wrinkles or bulges
Sharpen every crease before you collapse, and bring the three corners down together rather than one at a time.
The base is not square
One of the creases missed a corner. Unfold, correct it and collapse again.

When you’ll use it

The preliminary base is the gateway to traditional origami. Petal fold it on both sides and you have the bird base, the start of the crane and the flapping bird; squash and petal fold all four flaps and you have the frog base, the start of the lily and the sitting frog. Because it appears so often, it is worth learning both ways of making it: collapsing the square along creases made in advance, as in our Paper Crane, or folding a triangle in half and squashing each flap into a square. The collapse is faster; the squash method is easier to control with stiff paper.

Practice it on these models

Advanced15 min · 19 steps

Paper Crane

The origami crane is the traditional Japanese orizuru, a bird with spread wings folded from a single square without cuts or glue. It is an advanced model: expect about 15 minutes for your first one, mostly spent learning the petal fold and inside reverse fold.

Preliminary base: questions

Why is it called the preliminary base?+

It is the starting point for other bases: petal folding it gives the bird base, and squash folding its flaps leads to the frog base. It is also known as the square base.

What is the difference between the preliminary base and the waterbomb base?+

They use the same creases bent the opposite way. The preliminary base is a small square with its open corners at one end; the waterbomb base is a triangle.