The British Army of the 18th and 19th centuries theoretically operated on a system of "platoon firing", outlined in Sir Henry Dundas' Principles of Military Movement: the 1000-man battalion was split into ten units of 100 men each known as companies or platoons (the platoon was theoretically the administrative unit while the company was the tactical one; in practice both terms were used interchangeably), which when the battalion was deployed in line, would be deployed in three ranks, the first two firing, and the third to provide solidity to the whole line if it came to a melee, provide reserves, and pass reloaded muskets forward. The battalion would "fire by platoons", with each company (or two companies together, known as a grand division) firing individually, the aim being to create a continuous barrage while at least a quarter of the battalion's companies still had loaded muskets should something unexpected happen.

In practice, this would last a couple of volleys before the noise, the smoke and the fear led to firing degenerating into men loading and firing their muskets as fast as they could manage. If two battalion lines were caught in this, it would inevitably lead to a mounting trickle of casualties on both sides as they blazed away at each other until one side summoned the courage to launch a bayonet charge and sweep the enemy away. At Waterloo, two battalions of the Middle Guard – the 1er/3e Chasseurs and the 4e Grenadiers – successfully formed into line while facing Maitland's Guards Brigade of two battalions, the 2/1st and 3/1st Foot Guards. A long exchange of musket volleys followed, causing heavy casualties on both sides: From a starting strength of around 1,080 men on June 18th, Maitland's two battalions recorded 492 casualties, or 45% losses.

Similar experiences during the American Revolution had prompted a rethink in British infantry tactics. During the Peninsular War, battalions tended to dispense with the third rank entirely in favour of including it in the first and second ranks, thus increasing the number of muskets available to their battalion by 30%. Furthermore, it was accepted that the entire battalion firing at once to create a massive explosion of fire at close range, followed by a bayonet charge, was more effective in driving the enemy away and reducing casualties on your own side rather than getting into long musket duels. It was that coupled with the reverse slope defence that gave French columns so much grief in Spain and Portugal.

Sources:

Philip Haythornthwaite, British Napoleonic Infantry Tactics, 1792-1815

Paddy Griffith, French Napoleonic Infantry Tactics, 1792-1815

Paul K. Davis, Masters of the Battlefield: Great Commanders from the Classical Age to the Napoleonic Era