The line formation is a standard tactical formation

Tactical formation

A tactical formation is the arrangement or deployment of moving military forces such as infantry, cavalry, AFVs, military aircraft, or naval vessels...

 which has been used in Early modern warfare.
It continues the phalanx formation

Phalanx formation

The phalanx is a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar weapons...

 or shield wall

Shield wall

The wall, is a military tactic that was common in many cultures in the Pre-Early Modern warfare age...

 of infantry armed with polearms in use during antiquity and the Middle Ages.

The line formation provided the best frontage for volley fire, while sacrificing maneuverability and defense against cavalry. It came to the fore during the Age of Reason

Age of reason

Age of reason may refer to:* 17th-century philosophy, as a successor of the Renaissance and a predecessor to the Age of Enlightenment* Age of Enlightenment in its long form of 1600-1800* The Age of Reason, a book by Thomas Paine...

, when it was used to great effect by Frederick the Great and his enemies during the Seven Years' War

Seven Years' War

The Seven Years' War was a global military war between 1756 and 1763, involving most of the great powers of the time and affecting Europe, North America, Central America, the West African coast, India, and the Philippines...

.

An infantry battalion

Battalion

A battalion is a military unit of around 300–1,200 soldiers usually consisting of between two and seven companies and typically commanded by either a Lieutenant Colonel or a Colonel...

 would form "in line" by placing troops in several ranks, ranging in number from two to five, with three ranks being the most common arrangement. Each rank was approximately half a meter apart from the next, and soldiers in a rank were positioned closely to each other (usually within arm's length), with just enough room to present their weapons, fire, and reload. The line formation required that the troops be well-drilled and constantly supervised by officer

Officer (armed forces)

An officer is a member of an armed force or uniformed service who holds a position of authority. Commissioned officers derive authority directly from a sovereign power and, as such, hold a commission charging them with the duties and responsibilities of a specific office or position...

s and non-commissioned officer

Non-commissioned officer

A non-commissioned officer , called a sub-officer in some countries, is a military officer who has not been given a commission...

s (NCOs).

In 17th and 18th century European armies, NCOs were positioned to the rear of the line. They were equipped with long polearms, which they used to "dress" or arrange the ranks, a practice which included pushing down the weapons of any soldier who was aiming too high, as well as ensuring that the rank remained well-organized and correctly placed. Movement in line formation was very slow, and unless the battalion was superbly trained, a breakdown in cohesion was virtually assured, especially in any kind of uneven or wooded terrain

Terrain

Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

. As a result, line was mostly used as a stationary formation, with troops moving in columns

Column (formation)

A military column is a formation of soldiers marching together in one or more files in which the file is significantly longer than the width of ranks in the formation...

 and then deploying to line at their destination.

In addition, the line formation was extremely vulnerable to cavalry charges, from the flank

Flanking maneuver

In military tactics, a flanking maneuver, also called a flank attack, is an attack on the sides of an opposing force. If a flanking maneuver succeeds, the opposing force would be surrounded from two or more directions, which significantly reduces the maneuverability of the outflanked force and its...

s and rear, and these attacks usually resulted in the complete breakdown of cohesion and even destruction of the unit unless it was able to "form square

Infantry square

An infantry square is a combat formation an infantry unit forms in close order when threatened with cavalry attack.-Very early history:The formation was described by Plutarch and used by the Romans, and was developed from an earlier circular formation...

".

During the Napoleonic Wars

Napoleonic Wars

The Napoleonic Wars were a series of wars declared against Napoleon's French Empire by opposing coalitions that ran from 1803 to 1815. As a continuation of the wars sparked by the French Revolution of 1789, they revolutionised European armies and played out on an unprecedented scale, mainly due to...

, the British Army

British Army

The British Army is the land warfare branch of Her Majesty's Armed Forces in the United Kingdom. It came into being with the unification of the Kingdom of England and Scotland into the Kingdom of Great Britain in 1707. The new British Army incorporated Regiments that had already existed in England...

 famously adopted a thin two-rank line formation. This was adopted to compensate for their lack of numbers and to maximize their fire frontage. The British continued to use a two-rank line until the late 19th century. The famous Thin Red Line of the 93rd (Highland) Regiment at the Battle of Balaklava successfully held against a Russian cavalry attack, a rare occurrence.

A loose line formation is also used by many modern forces during assaults as it enables maximum firepower to be directed in one direction at once, useful when attacking an enemy position. It also enables the use of fire and movement

Fire and Movement

Fire and movement or fire and manoeuvre is a military tactic that uses suppressive fire, or threat thereof, to decrease the enemy's ability to return fire, organization and unit cohesion, and morale. The tactic is used by small unit commanders on the modern battlefield...

.

The line formation and cavalry

The line formation was also used by certain types of cavalry.

The Sassanid Persians, the Mamluks, and Muslim cavalry in India often used the tactics named "shower shooting". It involved a line of fairly well-armored cavalrymen (often on armored horses) standing in a massed static line or advancing in an ordered formation at the walk while loosing their arrows as quickly as possible by reducing their draw length.

In the 16th century, the heavy cavalry (reiters and cuirassiers) often attacked in a line formation. Later, dragoons began to use linear tactics, being on foot in the defense. Accordingly, the name "line cavalry" has moved from heavy cavalry to the dragoons. Hussars in the 15th-17th centuries wore armor, and often attacked in close line formation, but later hussars became a light cavalry and stopped using linear tactics. Cossacks never used linear tactics.