A   block  is a portable pulley, made of wood, metal, or wood and metal. It was originally a block of wood with a hole in it for a rope to reeve through. To save friction the hole was enlarged to take a pulley-wheel or sheave. Then the surplus wood was cut away from the outside of the block, leaving a wooden shell. This was grooved to take a rope strop spliced round the block to secure it in place as required. When the wooden shell split the block was useless. The next improvement was the 'iron-bound block' in which the strop or hook was riveted to an iron case carrying the sheave and its pin, the wooden shell (or cheek) acting as a fairing piece. Today the larger modern blocks are made of metal, and friction is further reduced by roller bearings between the sheave and its pin.

A  tackle  is a portable purchase consisting of a rope rove through two or more blocks in such a way that any pull applied to its hauling part is multiplied by an amount depending upon the number of sheaves in the blocks and the manner in which the rope is rove through them.

The blocks of a tackle are termed the standing block and moving block, and the rope rove through them is called the fall.

The amount by which the pull on the hauling part is multiplied by the tackle is called its mechanical advantage, and, if friction is disregarded, is equal to the number of parts of the fall at the moving block.
The number of parts at the moving block, and therefore the mechanical advantage, is always greater when the hauling part comes away from the moving block, and such a tackle is said to be rove to advantage. Conversely, a tackle in which the hauling part comes away from the standing block is said to be rove to disadvantage.

A  single whip  is a fall rove through a single standing block. No mechanical advantage is gained.

A  double whip  is the simplest form of purchase where a fall is rove through two single blocks, standing part being secured to the upper block. (a) and (b) above are two variations. Mechanical advantage is 2.

A  jigger  is a small tackle (smaller than a luff) used for general purposes.

A  handy billy  is a small tackle for general purpose

A  luff  is a purchase consisting of a double and a single block with the standing part made fast to the single block. Mechanical advantage is 4 or 3 depending whether it is rove to advantage or disadvantage.