Heart blocks are used for tensioning stays in the standing rigging. They are typically employed for the forestay, mainstay, and the preventer stay. The tension is achieved with a lanyard, rove either between two full heart blocks, or (for the forestay from about the second half of the 18th century) between one full heart block and one bowsprit heart block. Sizes (3–12 mm) correspond to the height of the full heart block. The same size code is also used for matching bowsprit heart blocks, ensuring correct pairing.
