Corrugated board is commonly rated two different ways, and they measure different things. The Mullen (or burst) test measures the force required to rupture the face of the board — historically tied to the board's resistance to puncture and rough handling. The Edge Crush Test (ECT) measures the edgewise compression strength of the board — how much force the board resists when compressed on its edge, which relates to the stacking strength of a box. Both appear on box certificates and specifications. They are not interchangeable: a board specified by burst and a board specified by ECT are characterized on different properties, which is why specifications state which basis is being used. This post simply describes what each rating measures; which rating a particular application calls for is a design decision made for that application.