Blocks act as pulleys used in conjunction with the miles of line that make up the running rigging. They multiply the force exerted by the crew, enabling yards and sails to propel ships 2000 tons or larger. The more parts a tackle has (rope "legs" between two blocks, determined by the number of sheaves in the blocks), the more the motivating force is multiplied, and the heavier the object which can be moved.
Blocks came in many sizes and of one, two, three, even four sheaves. Note: Model blocks do not have scale sheaves. Instead, sheaves are represented by a hole in the block.