Builder's Old Measurement in the context of Slave Trade Act 1788

A cog is a type of ship that was used during the Middle Ages, mostly for trade and transport but also in war. It first appeared in the 10th century, and was widely used from around the 12th century onward. Cogs were clinker-built, generally of oak. Cogs were fitted with a single mast and a single square sail. They were used primarily for trade in north-west medieval Europe, especially by the Hanseatic League. Typical seagoing cogs were from 15 to 25 meters (49 to 82 ft) long, 5 to 8 meters (16 to 26 ft) wide, and were of 30–200 tons burthen. Cogs were rarely as large as 300 tons although a few were considerably larger, over 1,000 tons.

Although the name cog is recorded as early as the 9th century, the seagoing vessel of that name seems to have evolved on the Frisian coast during the 12th century. Cogs progressively replaced Viking-type vessels such as knarrs in northern waters during the 13th century. Cogs could carry more cargo than knarrs of a similar size. Their flat bottoms allowed them to settle on a level in harbour, making them easier to load and unload. Their high sides made them more difficult to board in a sea fight, which made them safer from pirates.

A gun-brig was a small brig-rigged warship that enjoyed popularity in the Royal Navy during the Napoleonic Wars, during which large numbers were purchased or built. In general these were vessels of under 200 tons burthen, and thus smaller than the more common Cherokee-class brig-sloops or the even larger Cruizer-class brig-sloops. The gun-brigs generally carried 12 guns, comprising two long guns in the chase position and ten carronades on the broadsides.

For brig-rigged sloops, see List of corvette and sloop classes of the Royal Navy. For gunboats, see List of gunboat and gunvessel classes of the Royal Navy.