Almost all of the diagrams that I have come across in my book show both valley folds and mountain folds. To someone who has never held a square piece of paper before, these two seem very similar. Both create a crease, but they crease the paper in opposite directions. A valley fold folds the paper inward and makes a V-shape. A mountain fold is the opposite direction, and makes an inverted V-shape.
It is important to differentiate between them, because it will help you understand the following diagrams easier.
Try taking a regular square piece of paper, fold it in half. Now open it back up and put the paper flat on the table. Push the sides up and the middle of the crease down, you have just done a valley fold. Flip the paper over, you should now see the same crease. However, since you are looking at it from the other side, the crease is now in between the two sides making a mountain fold. So you can see that there was no change to the crease itself, just the direction you are looking at it.
You will notice that diagrams usually show these folds using different line patterns and arrows. A dashed line tends to mean a valley fold, and a dash-dot line usually stands for a mountain fold. Because the exact graphic style can vary a bit from one diagram to the next, paying attention to the arrows and surrounding steps is important too. Be sure to know which portion of the paper is moving, if it has been flipped over, and where the folded portion should end up. This is especially important after rotations, because a valley fold on one side will become a mountain fold on the other side, or vice versa.
To practice, you can use any square piece of paper and fold it horizontally. Then unfold it and try to fold it both ways, as a valley and a mountain. You should see that the paper follows the crease more easily one way than the other. Do the same with a diagonal crease. This will also teach you about pre-creases. Sometimes you will have to precrease the paper a certain way, and use it in the opposite direction later in the fold sequence. You can see this in reverse folds, squash folds, and in models with multiple layers.
I find most mistakes happen when people press too hard. They see a fold line, assume they know where the flap needs to go, and immediately crease the paper sharply. After they make a hard crease in the wrong direction, you can usually reopen the paper and correct it, although too many hard creases can damage thin origami paper. A good way to avoid this mistake is to first move the flap gently into its new position, then press from the center outwards. It’s also a good idea to align the edges or intersections to ensure accuracy.
Another thing that can help you differentiate between the two is by studying the crease patterns of completed models. Take a simple model, unfold it, and see how the mountain folds and valley folds work together to make the finished model. Notice how some of the creases point one way, while others point the other way. Look at the crease pattern and see how many mountain folds and valley folds there are. Don’t think of the crease pattern as a mess of lines, but instead think of how each crease was created by changing its direction during the construction process. This will help you understand reverse folds, which involve converting valley folds to mountain folds, and vice versa.
The next time you read a diagram, don’t just look at the fold line. Look at the arrow, the orientation, the layer that moves, and the final placement of the edge. If you can stop for a moment, analyze the paper, and determine “this folds inward as a valley” or “this folds outward as a mountain”, then you are reading the paper, not simply copying a picture. This subtle change will provide you a much stronger base when constructing longer folding sequences.
