A spreader is 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...

 on a sailboat

Sailboat

A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in the size are not strictly defined and what constitutes a sailing ship, sailboat, or a...

 used to deflect the shrouds

Shroud (sailing)

On a sailboat, the shrouds are pieces of standing rigging which hold the mast up from side to side. There is frequently more than one shroud on each side of the boat....

  to allow them to better support the 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...

. Often, there are multiples, called spreaders. The spreader or spreaders serve much the same purpose as the crosstrees

Crosstrees

Crosstrees are the two horizontal struts at the upper ends of the topmasts of sailboats, used to anchor the shrouds from the topgallant mast. Similarly, they may be mounted at the upper end of the topgallant to anchor the shrouds from the royal mast .See tops for the description of their purpose....

 and tops

Top (sailing ship)

On a traditional square rigged ship, the top is the platform at the upper end of each mast. This is not the masthead "crow's nest" of the popular imagination – above the mainmast is the main-topmast, main-topgallant-mast and main-royal-mast, so that the top is actually about 1/4 to 1/3 of the way...

in a traditional sailing vessel.

Spreader design and tuning can be quite complex. The spreaders may be fixed (rigid) or swinging (pivoted at the mast). The purpose of the spreaders is to control, by either limiting, or inducing, bend into the spar so that when the windward shroud is loaded the mast achieves the desired bend characteristics. The spreaders may be designed to be angled in a way that either forces bend in to the spar or reduces bend, depending on the desired results.