Foretriangle

The approximately triangular area bounded by the mast, forestay and foredeck of a sailing vessel.

The foretriangle defines the space in which a conventional headsail is set. Its principal dimensions are commonly represented by I, the vertical height from the relevant deck or sheer reference to the forestay attachment, and J, the horizontal distance from the mast to the forestay’s deck attachment.

These measurements are used in sail design and rating calculations. A headsail may overlap aft of the mast, so its actual area is not necessarily limited to the geometric area of the foretriangle.