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The extra degree of motion provided by the use of a toggle jaw on a turnbuckle allows for minor self-alignment, lessening the strain on other rigging components, such as chainplates and swages, especially when subjected to momentary high stresses - like someone falling against a shroud. According to Master Rigger Brion Toss, while any stay or shroud can benefit from having a toggle at its lower attachment point, they are especially critical to use on both ends of head and forestays, which must withstand substantial side loading from sails.
Note: Length is measured from bottom of swage hole to center of clevis pin.