A unit arrayed in an Open Order formation closely resembles one in a Close Order formation; the key differences lie in how the unit moves and interacts with terrain. As with a unit in Close Order, a unit in Open Order consists of two or more models that are arranged in base contact with each other, edge-to-edge and front corner to front corner, as shown in Fig 182.1.

All models in such a unit must face the same direction. In addition, all models in the unit must be arranged in a formation that consists of one or more horizontal rows, called ranks, and a number of vertical rows, called files.

As far as possible, there must be the same number of models in each rank. Where this is not possible, it must be the rear rank that has fewer models.