Ratlines

Ratlines, pronounced "rattlin's", are lengths of thin line tied between the shrouds of a sailing ship to form a ladder. Found on all square rigged ships, whose crews must go aloft to stow the square sails, they also appear on larger fore-and-aft rigged vessels to aid in repairs aloft or conduct a lookout from above.
Lower courses in a ratline are often made of slats of wood (battens) for support where the distance between shrouds is greatest. These wooden boards are called rat-boards. In some instances holes in these slats guide and organise low-tension lines between the deck and the rig.