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Meaning of twofold purchase

two'fold pur'chase

Pronunciation: [key]
— Mach., Mach.,Mech.
  1. a purchase using a double standing block and a double running block so as to give a mechanical advantage of four or five, neglecting friction, depending on whether the hauling is on the standing block or the running block.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.