A fine-thread castle nut in accordance with DIN 979 has a crown-shaped top, which is used for locking together with a cotter pin. The nut has a lower thread pitch than normal ("tighter" threads), which helps reduce loosening of the fastener over time. The electro-zinc plated surface treatment gives the nut a bright and clean finish, but it is still intended for indoor use. It is typically used in applications where a low profile and a secure fastening are required.