Use swivel bases when leads must rotate to face the trimmer. Ball bearing swivel bases feature dual rows of Delrin ball bearings to ensure that they swivel freely even under high loads. The bases include a stop that prevent the base from swiveling completely and fouling lines. Bases include stand-up springs and a U-adapter to accept a variety of appropriate blocks Use for mainsheets, jib sheets, and control lines Models 205, 402 are suitable for 57 mm (2-1/4") blocks Models 144 is suitable for 57 mm (2-1/4") or 75 mm (3") blocks Model 1574 is suitable for 76 mm (3") midrange blocks Note: models 238, 239, 379 do not swivel