A cisterna (plural cisternae) refers to a flattened membrane disk that makes up the Golgi apparatus. A Golgi stack may contain anywhere from three to twenty cisternae but most contain about six. Golgi cisternae can be separated into four classes; cis, medial, trans, and TGN (trans-Golgi network). Each cisternae class contain different enzymes working within it. This helps to prevent any two reactions form competing. Almost all cisternae in the Golgi apparatus present multiple stacking. There are only eight instances in which the stacked characteristic has been lost.