Historically, the phalanx
was a tactical formation used by warriors in many ancient
armies, particularly the greek hoplites.
The men in a phalanx would form serried ranks, with the
front rank locking shields and the men behind pushing them
forward. This formation would present a veritable wall of
shields and spear points to smash into the enemy. The phalanx
was very effective when facing an enemy to the front, but
was vulnerable to flank attacks.
In 40k terms, a "phalanx" is a formation type
primarily used by infantry heavy armies. It generally consists
of a solid block of troop types, where every element of
the army can both give and receive support from the others.
The phalanx formation relies heavily on the concept of attrition
warfare, using superior numbers/resilience to grind
down your opponent. There are several variations on this
theme, often associated with different armies and play styles.
Here are some basic variations on the phalanx formation.