Crease (fold and unfold)
A crease, or fold-and-unfold, is a fold that you make firmly and then open again so the line stays in the paper as a guide for later steps.
Fold as normal, press the line sharply, then open the paper back to where it was. Diagrams show this with a fold line and an arrow that goes out and comes back, or with a thin line indicating an existing crease. These guide lines are used later for alignment or for collapsing a base.
Weak creases are the main cause of trouble in later steps. Press each crease firmly with a fingernail along its full length. Also check that crease lines run exactly corner to corner or edge to edge, because small errors grow as the model develops.
