A two-decker is a sail warship

Warship

A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way from merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships...

 which carried her guns on two fully armed decks. Usually additional guns were carried on the upper works (forecastle

Forecastle

Forecastle refers to the upper deck of a sailing ship forward of the foremast, or the forward part of a ship with the sailors' living quarters...

 and quarterdeck

Quarterdeck

The quarterdeck is that part of a warship designated by the commanding officer for official and ceremonial functions. In port, the quarterdeck is the most important place on the ship, and is the central control point for all its major activities. Underway, its importance diminishes as control of...

), but this was not a continuous battery, so were not counted.

Two-deckers ranged all the way from the small 40-gun fourth-rate

Fourth-rate

In the British Royal Navy, a fourth rate was, during the first half of the 18th century, a ship of the line mounting from 46 up to 60 guns. While the number of guns stayed subsequently in the same range up until 1817, after 1756 the ships of 50 guns and below were considered too weak to stand in...

 up to 80- or even 90-gun ships, with the third-rate

Third-rate

In the British Royal Navy, a third rate was a ship of the line which from the 1720s mounted between 64 and 80 guns, typically built with two gun decks . Years of experience proved that the third rate ships embodied the best compromise between sailing ability , firepower, and cost...

 or "seventy-four

Seventy-four (ship)

The "seventy-four" was a type of two-decked sailing ship of the line nominally carrying 74 guns. Originally developed by the French Navy in the mid-18th century, the design proved to be a good balance between firepower and sailing qualities, and was adopted by the British Royal Navy , as well as...

" being the archetype.