A purchase is a mechanical device by means of which an applied pull or force is increased; it may be a system of levers, a system of revolving drums or wheels geared to one another, or a combination of blocks or pulleys rove with rope or chain.
A tackle (pronounced ‘ taycle ‘) is a purchase consisting of a rope rove through two or more blocks in such a way that any pull applied to its hauling part is increased by an amount depending upon the number of sheaves in the blocks and the manner in which the rope is rove through them.
