Valley fold
Dashed line: where to fold. Arrow with a solid head: which way the paper moves.
Fold the paper toward you so the crease sinks like the floor of a valley. The most common fold in origami.
HistoryThe dashed valley line comes from the diagram system of Japanese master Akira Yoshizawa, published in 1954. Samuel Randlett and Robert Harbin adopted it in their English books in the 1960s, and it became the world standard: the Yoshizawa–Randlett system.