Valley and mountain fold


Post is a part of a larger series (Folds):

How many folds exist in the origami. If you survey the internet, you will find out that there are many of them (Valley fold, Mountain fold, Inside reverse fold, Outside reverse fold, Rabbit ear, Squash fold, Swivel fold, Patel fold, Open sink, Closed sink, Unsink fold etc.)

But, the truth is that there are only two folds. Everything else belongs to the category of specific folding techniques that when broken down into the fundamental elements reduce to just two basic folds:

  • Valley fold and
  • Mountain fold

Valley and mountain fold definition

A Valley fold is in its essence a paper fold that forms a trench. A Mountain fold is, on the other hand, a paper fold that forms a ridge (look at figure 1). And, that’s it. There is nothing more to it.

Simple example of valley and mountain fold.
Figure 1

What’s even more interesting, both mountain and valley fold are in fact the same fold. If we turn this paper upside down, what was a valley will now be a mountain, and of course, what was a mountain will now be a valley. So, there is no real difference.

Orientation

To prove you that there are really only two basic folds, let’s think for a moment, how in these days origami models’ instructions are shown. You see, in modern-day origami, the vast majority of models (especially more advanced ones) are shown only in the form of a crease pattern. Diagrams are exceptionally rare.
For those who do not know, the crease pattern is a diagram that shows all creases that are needed to fold an origami model, rendered on a single image. These creases include hinge, ridge and axial creases.
And that’s it. There are no additional instructions. Only what you got is sort of a plan of a model.

In figure 2 you can see an example of a crease pattern.

Traditional origami paper crane and something that resembles a crease pattern.
Figure 2: Crease pattern of a traditional crane

Unfortunately, the crease pattern in figure 2 is not a real crease pattern. You see, for this to be a real crease pattern, all creases must have clearly defined orientation. In other words, for every crease in the crease pattern, it must be clearly shown if it is a mountain or a valley fold. Remember this, orientation is a term that defines if a crease is a mountain or a valley fold.

If it is so, let’s draw a proper crease pattern. But, before we do that, let’s define a convention. For example, the valley folds could be marked with a blue line while the mountain folds could be marked with a red line.

Proper crease pattern of a traditional origami paper crane (version 1)
Figure 3

Of course, we could do this differently since it is only a convention, an agreement. For example, some origami authors tend to mark the mountain folds with the chain line (dot alternating with a dash), and the valley folds with the dashed line.

Proper crease pattern of a traditional origami paper crane (version 2)
Figure 4

Again, these two are not the only possible approaches. If you want you can mark mountain and valley folds in your own distinct way. Only what is important is to be consistent and to stick to the convention, even if the convention is your own.

Final words

Now that I have shown you how a crease pattern should look, I hope you understand why I have told you that there are only two basic origami folds.

After all, if you look at figure 3, do you see anything except valley and mountain folds? I don’t think so.