The internal support structure of cardboard makes it difficult to create an even bend. Breaking a piece of cardboard's internal spine allows the material to form a bend. To break a piece of cardboard's spine and produce an even bend, you need to score the cardboard. Scoring cardboard refers to compressing the internal support system of the material without creating a cut. Cutting through the cardboard weakens its structure and causes it to tear at the bend point.
Lay a piece of cardboard flat on a work table. Mark the bend at two points on the face of the cardboard with a pencil.
Align one side of a straightedge with the two marks representing the bend location.
Retract the ballpoint of a retractable pen. Set the end of the pen on the surface of the cardboard with one side of the pen sitting against the aligned side of the straightedge.
Press the end of the pen toward the surface of the cardboard. Pull it along the straightedge. Repeat the process of pulling the pen along the straightedge until the cardboard has a visible indentation.
Remove the straightedge from the surface of the cardboard. Bend the cardboard at the pen indentation. Replace the straightedge and continue the scoring process if the cardboard requires excessive pressure to create the bend or the bend is not uniform along its length.