If you look at the outer hull of commercial ships, you might find a painted circle bisected with a long horizontal line. This marking is called the load line, or as I prefer, the Plimsoll line. This simple graphic design has saved thousands of lives. The Plimsoll line shows the maximum loading point of the ship and lets a third party know, plainly and clearly, when a vessel is overloaded and at risk of sinking in rough seas. If you see that horizontal line above the water, you’re good, if you don’t, you could be sunk. The load line was named after the crusading British MP Samuel Plimsoll. The advent of insurance in the 19th century, created an incentive for ship owners to purposely sink their own ships and collect the insurance money. This grim practice became so widespread, and killed so many merchant seamen, that the over-insured, overloaded vessels became known as “coffin ships.” Samuel Plimsoll (“the sailors friend”) fought for sweeping merchant shipping regulation that led to the adoption of the marking that bears his name. Tristan Cooke, a human factors engineer and creator of a great blog called Humans in Design, tells us the history of the Plimsoll line and explains why it’s one of his favorite examples of design.