In moving by wings, they formed four columns, two of cavalry and two of infantry: in moving by a flank, they formed only two very long columns; the cavalry, however, sometimes formed a third and separate column in flank movements, but this disposition was rarely made.
The French Revolution introduced the system of grand divisions composed of the four arms combined; each division moved separately and independently of the other.

In the wars of the Empire, Napoleon united two or more of these divisions into a _corps d'armee,_ which formed a wing, the centre, or reserve of his grand army.

In addition to these divisions and _corps d'armee,_ he had large reserves of cavalry and artillery, which were employed as distinct and separate arms.
If the forces be sufficiently numerous to fight by _corps d'armee,_ each corps should have its own reserve, independent of the general reserve of the army.

Again, if the forces be so small as to act by grand divisions only, each division should then have _its_ separate reserve.
An army, whether composed of separate corps or of grand divisions, usually forms, on the field of battle, a centre, two wings, and a reserve.

Each corps or division acts by itself, with its infantry, cavalry, artillery, and engineer troops.