The primary source of sail power on a sailing vessel is usually the mainsail. There are exceptions when a lesser sail (such as a jib sail) is utilized and the mainsail is lowered. In such cases, the definition doesn’t apply since the primary source of power can only be the sail in use, but in most cases the definition works. Additionally, the mainsail is always affixed to the main mast and either extends perpendicular to the vessel (extending to the left and right of the main mast) or lengthwise (above and parallel to the vessel) toward the aft (or rear) section, or some angled variation thereof.
In ancient history, the mainsail was made up of a patchwork of wool. This material was effective in harnessing wind but ultimately stretched and sagged making the sail increasingly ineffective. Eventually, various types of patch worked linens were utilized until modern technology made seamless, one-piece nylons and other man made materials possible. Today’s popular materials include Mylar® and Kevlar®. These modern materials not only provide lightweight custom shapes, they also include shape retention properties to minimize distortions.
A main sail can be a square sail, a triangular sail or some varied shape of the two, as found in lateen or gaff sails. Unless referring to a square-rigged mainsail — rarely seen today — which is attached to a yardarm that intersects the main mast, the front edge — commonly called the luff — of a modern mainsail attaches to the main mast and the bottom of the sail — commonly called the foot — attaches to the boom. The boom is the horizontal beam attached to the main mast and is exclusively designed for supporting the bottom of the mainsail on a modern sailboat.
Like all sails on a sailboat, the position of the mainsail in relationship to the wind direction and where the vessel is headed is known as the angle of sail. Each angle is understood as a "point of sail" and it constantly changes throughout a sailing voyage. There are three basic points of sail though there are multiple variations of each. Whenever a sailboat is sailing into the wind it will utilize a "beating" point of sail. Whenever the wind is from the side of a sailboat (known as abeam), the point of sail required is known as "reaching." Whenever the wind comes from the rear of (or astern) the vessel, the point of sail is known as "running." Recognizing these points of sail are critical in determining how to properly position (trim) the mainsail.