The concept of rating a ship was originated in the 17th century, after the galleons Era.

Warships were divided according to the number of guns carried into classes or rates.

The rating system was changing with the years and nations, acording to navies and duties.

A 60 guns ship in the 1720's was considered a "ship of the line" but after the 1760's had not enough firepower to stand in the line of battle.

There were six "universal" main rates, slightly different depending of the ship's nationallity.

Per example, a french 80 was considered a 2nd rate ship, because of his heavy guns, almost equal to a british 98.

Also, duties of ships were assigned upon his rate (guns, ship size, speed and depth).

We are going to use mostly these six main rates to classify ships.