Mountain fold and valley fold are two basic folds of origami. All other folds being basic variations of these two folds.The definition of mountain and valley folds is related to the side of the paper that is initially facing you,because when you invert the paper that had made mountain fold ，the mountain folding turn to valley folding.
With a Mountain Fold the crease bends the paper down and the crease resembles a mountain.In origami instructions, arrows are usually used to indicate the direction of folding and dashed lines to indicate where the creases is.When a piece of paper has made mountain fold，The dashed line used to indicate where the creases is,will be sitting the top of the mountain.
Step 1
: Prepare a sheet of paper.Fold the right half of the paper behind itself along the dotted line.
Step 2
: Use its two edgs as fulcrums to make it stand up.Now the paper look like a mountain.The dashed line used to indicate where the creases is,will be sitting the top of the mountain.
With a Valley Fold the crease bends the paper up and the crease resembles a valley.The paper should form ‘V’.In origami instructions, arrows are usually used to indicate the direction of folding and dashed lines to indicate where the creases is.When a piece of paper has made a valley fold，The dashed line used to indicate where the creases is,will be at the bottom of the valley.
Step 1
: Prepare a sheet of paper.Fold the left edge of the paper to right edge along the dotted line.
Step 2
: Use its center line as fulcrums to make it stand up.Now the paper should form V”.The dashed line used to indicate where the creases is,will be at the bottom of valley.
When you understand mountain folds and valley folds, you can try to fold the Origami fan, it is simple enough and completely composed of mountain folds and valley folds, you will have a deeper understanding of mountain folds and valley folds.
