What defines a Ship-of-the-line?

A Ship-of-the-line is specifically defined as a type of naval warship that was prominent from the 17th to the mid-19th century, characterized by its heavy armament, typically ranging from 64 to 100 guns, which positioned it to engage in battle lines with other ships. These vessels were built for the primary purpose of naval warfare and were designed to withstand and deliver heavy firepower during battles. Their large size and firepower allowed them to play a central role in naval battles, forming the backbone of a fleet.

The other options do not correctly define a Ship-of-the-line. The fast ships with limited armament refer to different classes of vessels, such as frigates, which were built for speed and maneuverability rather than heavy combat. Small vessels for coastal defense would typically include patrol boats or other specialized ships designed for specific missions close to shore, lacking the defining characteristics of a Ship-of-the-line. Lastly, merchant ships are primarily designed for trade and transport rather than combat and do not fit the historical or functional profile of a Ship-of-the-line.