The models that make up your Warhammer army must be organised into ‘units’. A unit usually consists of several models that have banded together, but a single, powerful model such as a lone character, a chariot or a Dragon, a war machine and its crew, and so on, are also considered to be a unit.
A unit consists of 1 or more models that are arranged in orthogonal base contact with each other, which is a fancy way of saying ‘edge-to-edge and front corner to front corner’. See the diagrams below – it’s much clearer as a picture than it is in words.
All models in 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 lines, called ranks and a number of vertical lines, called files. This is why we often refer to basic warriors as ‘rank and file’ troops.
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. Once formed into a unit, the models move and fight as a single entity for the rest of the battle.
Here, you can see examples of correct and incorrect formations.