A ship of the line was a type of naval warship constructed from the 17th through to the mid-19th century to take part in the naval tactic known as the line of battle, in which two columns of opposing warships would manoeuvre to bring the greatest weight of broadside firepower to bear. Since these engagements were almost invariably won by the heaviest ships carrying the most powerful guns, the natural progression was to build sailing vessels that were the largest and most powerful of their time.
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you can find more on - https://en.wikipedia.org/wiki/Ship_of_the_line
