A mainsail is a sail located behind the main mast

Mast (sailing)

The mast of a sailing vessel is a tall, vertical, or near vertical, spar, or arrangement of spars, which supports the sails. Large ships have several masts, with the size and configuration depending on the style of ship...

 of a sailing vessel.

On a square rig

Square rig

Square rig is a generic type of sail and rigging arrangement in which the primary driving sails are carried on horizontal spars which are perpendicular, or square, to the keel of the vessel and to the masts. These spars are called yards and their tips, beyond the last stay, are called the yardarms...

ged vessel, it is the lowest and largest sail on the main mast.

On a fore-and-aft rig

Fore-and-aft rig

A fore-and-aft rig is a sailing rig consisting mainly of sails that are set along the line of the keel rather than perpendicular to it. Such sails are described as fore-and-aft rigged....

ged vessel, it is the lowest and largest and often the only sail rigged aft of the main mast, and is controlled along its foot by a spar

Spar

In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a...

 known as the boom

Boom (sailing)

In sailing, a boom is a spar , along the foot of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail. The primary action of the boom is to keep the foot of the sail flatter when the sail angle is away from the centerline of the boat. The boom also serves...

. A sail rigged in this position without a boom is generally called a trysail

Trysail

A trysail is small triangular or square fore-and-aft rigged sail hoisted in place of a larger sail when winds are very high....

, and is used in extremely heavy weather.

The modern Bermuda rig

Bermuda rig

The term Bermuda rig refers to a configuration of mast and rigging for a type of sailboat and is also known as a Marconi rig; this is the typical configuration for most modern sailboats...

 uses a triangular mainsail as the only sail aft

Aft

Aft, in naval terminology, is an adjective or adverb meaning, towards the stern of the ship, when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?"...

 of the mast, closely coordinated with a jib

Jib

A jib is a triangular staysail set ahead of the foremast of a sailing vessel. Its tack is fixed to the bowsprit, to the bow, or to the deck between the bowsprit and the foremost mast...

 for sailing upwind. A large overlapping jib or genoa

Genoa (sail)

The genoa or jenny was originally referred to as the 'overlapping jib' or the Genoa jib, being named after the city of Genoa as explained below. It is a type of large jib or staysail used on bermuda rigged craft that overlaps the main sail, sometimes eliminating it. It is used on single-masted...

 is often larger than the mainsail. In downwind conditions (with the wind behind the boat) a spinnaker

Spinnaker

A spinnaker is a special type of sail that is designed specifically for sailing off the wind from a reaching course to a downwind, i.e. with the wind 90°–180° off the bow. The spinnaker fills with wind and balloons out in front of the boat when it is deployed, called flying. It is constructed of...

 replaces the jib.

Most modern mainsails are "full-batten" mainsails. Batten

Batten

A batten is a thin strip of solid material, typically made from wood, plastic or metal. Battens are used in building construction and various other fields as both structural and purely cosmetic elements...

s enable the mainsail to project farther away from the mast. However, there is some cost associated with the battens themselves, "batten pockets" need to be sewn into the sail, and "batten cars" are needed to allow the sail to be raised and lowered.

Traditional fore-and-aft rigs used a four-sided gaff rig

Gaff rig

Gaff rig is a sailing rig in which the sail is four-cornered, fore-and-aft rigged, controlled at its peak and, usually, its entire head by a spar called the gaff...

ged mainsail, sometimes setting a gaff topsail

Topsail

A topsail is a sail set above another sail; on square-rigged vessels further sails may be set above topsails.- Square rig :On a square rigged vessel, a topsail is a square sail rigged above the course sail and below the topgallant sail where carried...

 above it.

A roll mainsail is furled by being rolled

Roller furling

Roller furling is a method of furling or reefing wherein a sail is rolled around a stay or rotating spar. Roller furling is most commonly encountered on foresails, such as jibs, with mainsails a distant second.-Furling methods:...

 within (or around) the mast or boom.

Before Nathanael Greene Herreshoff invention of sail tracks and slides in the 1880s, mainsails were limited in height. Traditional mainsails were held against the mast by hoops that went the full way around the mast. This meant a traditional mainsail could be raised no higher than the first point a rope or wire was required to keep the mast upright.