This is a great question, and one that isn't perhaps as simple as we might think. Between 1600 and 1815, soldiers fought in a variety of linear depths. Formations in the early period (1600s) were often 6 men deep, while by 1750, armies were moving toward 3, or occasionally even 2, deep lines.

Throughout the entire period, commanders weighed the need for their formations to retain mass vs. the need to produce a large volume of fire. In the 1600s, soldiers using matchlock muskets still employed the countermarch, that is, you shot when you were at the front of the formation, and then walked to the back of the formation, so the following man in the formation could then shoot. By the 1700s, officers had abandoned this for each man loading in their own place in the formation, and in order to maximize the number of men who could shoot at any one time, formations shallowed out to 4, 3, and 2 men deep.

The exact arrangement of the men depended on the army and war in question. In the British Army during the American War of Independence, (a remarkably flexible army) soldiers stood in two ranks, in open order, and the first rank did not kneel in order to fire.

In the Prussian Army during the Seven Years War, men usually employed a three rank line, and the first rank kneeled, the second rank hunched over a small amount, and the third rank took a step offsetting: this is called the process of locking on.

It is also important to note that armies were flexible enough to adopt different depths of formation depending on the specific circumstances they were faced with. A Swiss military observer in Prussia, Franz Haller, noted:

Apart from the customary formation in three ranks, (for the old practice of putting them on four had been almost continuously abolished before the [War of Austrian Succession]), the Prussian infantry…had also been trained in two ranks; this maneuver took place especially in the years of the Seven Years' War, when the Prussians, in order to hide their weakness, often marched on only two lines; yes, in the camp near [Strehla] in 1760, General Hülsen was forced to place 5 battalions from his left wing... only one rank deep, in order to cover the whole front of this position.[1]

If the men were arranged in three ranks, it was customary to have the front rank kneel, the front men would kneel during the preparatory commands to fire ("make ready") and stand when they had been ordered to reload after firing. When in two, or one, ranks, the men would often not kneel. Many military theorists decried the practice of allowing the men of the front rank to kneel, because they often were timid in standing back up, as a result of misfires from men in the rear ranks.

Let me know if you have any follow up questions.

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For more on formation depths in eighteenth-century armies, see:

Christopher Duffy, Military Experience in the Age of Reason (Athaneum: 1988)

Matthew Spring, With Zeal and with Bayonets Only (Oklahoma University Press: 2008)

[1] Franz Ludwig Haller, Militärischer Charakter und merkwürdige Kriegsthaten Friedrich des Einzigen, (Berlin: 1796) 12.