In origami valley-fold, the fold done in the paper is such that it lays over the top of the surface of the paper.
The valley-fold is usually referred to as the “fold”.
When unfolded, the fold sinks along the crease which resembles a “river valley”. Hence, called valley-fold.
The two basic origami folds are the valley-fold and the mountain-fold. The valley-fold is the most used fold in making origami models. The mountain-fold is the opposite of the valley-fold.
Note: Valley-fold is folded horizontally, vertically, diagonally etc.
Example of a valley fold
Step 1: Position the origami paper.
Step 2: Fold the paper diagonally at the top-left edge. Crease the paper to form a valley-fold.
Why is it called valley-fold?
Step 1: Take an origami paper.
Step 2: And, fold the left edge of the paper to meet the right edge. Then crease the fold.
Step 3: Unfold the fold made in Step 1. The crease along the unfolded fold resembles a valley. Hence, called valley-fold.