Blocks are of two kinds, _made_ and _morticed_. A _made block_ consists of four parts,--the _shell_, or outside; the _sheave_, or wheel on which the rope turns; the _pin_, or axle on which the wheel turns; and the _strap_, either of rope or iron, which encircles the whole, and keeps it in its place. The sheave is generally strengthened by letting in a piece of iron or brass at the centre, called a _bush_.
A MORTICED BLOCK is made of a single block of wood, morticed out to receive a sheave.
All blocks are single, double, or three-fold, according to the number of sheaves in them.
There are some blocks that have no sheaves; as follows: a _bull's-eye_, which is a wooden thimble without a sheave, having a hole through the centre and a groove round it; and a _dead-eye_, which is a solid block of wood made in a circular form, with a groove round it, and three holes bored through it, for the lanyards to reeve through.
A SISTER-BLOCK is formed of one solid piece of wood, with two sheaves, one above the other, and between the sheaves a score for the middle seizing. These are oftener without sheaves than with.
SNATCH-BLOCKS are single blocks, with a notch cut in one cheek, just below the sheave, so as to receive the bight of a fall, without the trouble of reeving and unreeving the whole. They are generally iron-bound, and have a hook at one end.
A TAIL-BLOCK is a single block, strapped with an eye-splice, and having a long end left, by which to make the block fast temporarily to the rigging. This tail is usually selvageed, or else the strands are opened and laid up into sennit, as for a gasket.
A TACKLE is a purchase formed by reeving a rope through two or more blocks, for the purpose of hoisting.
A WHIP is the smallest purchase, and is made by a rope rove through one single block.
A GUN-TACKLE PURCHASE is a rope rove through two single blocks and made fast to the strap of the upper block. The parts of all tackles between the fasts and a sheave, are called the _standing parts_; the parts between sheaves are called _running parts_; and the part upon which you take hold in hoisting is called the _fall_.
A WHIP-UPON-WHIP is where the block of one whip is made fast to the fall of another.