Running backstays support the headstay in a fractionally rigged boat. A masthead rig has the advantage of not needing them. A running backstay runs from each lateral corner of the stern

Stern

The stern is the rear or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite of the bow, the foremost part of a ship. Originally, the term only referred to the aft port section...

 to the mast at the level where the forestay

Forestay

On a sailing vessel, a forestay, sometimes just called a stay, is a piece of standing rigging which keeps a mast from falling backwards. It is attached either at the very top of the mast, or in fractional rigs between about 1/8 and 1/4 from the top of the mast. The other end of the forestay is...

 begins in the fractional rig. Together with other rigging

Rigging

Rigging is the apparatus through which the force of the wind is used to propel sailboats and sailing ships forward. This includes masts, yards, sails, and cordage.-Terms and classifications:...

, it supports 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...

. Because they are attached low on mast, they can present a significant problem in an accidental gybe, as the boom hits the stay, with the possibility of breaking the boom, mast, or both.

During beating or reaching, the running backstay in windward side is in tension. The one in the leeward side is lazy. As the boat tacks they change over. Another function of the running backstay is to adjust the tension of the forestay to suit the sailing angle and sea condition. In general, during a hard beat when the boat sails very close to the wind, the running backstay is tightened to increase the tension of the forestay. The 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 sheeted in. Under these conditions, the draft of the genoa is brought forward reducing the drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

.