Backstay
F5. The Reference Layer: Glossary, Units, Signals and Information SourcesDefinition
Standing rigging supporting a mast from aft.
A backstay is standing rigging that runs from the masthead (or an upper point on the mast) aft to the stern or quarters, holding the mast against forward-bending loads from the headsails and forestay. Permanent backstays are fixed; running backstays are tensioned on the windward side and eased to leeward when tacking. On Bermudan sloops a single masthead backstay opposes the forestay; fractional rigs add running backstays or check stays to control mast bend and headstay tension.
Source: Standard rigging terminology; Dave Gerr, The Nature of Boats, and Skene's Elements of Yacht Design (Kinney).