The mainmast is the tallest and most important mast on a sailing ship. A mast is a tall vertical pole that holds up the sails, and on ships with multiple masts, the mainmast is typically the one in the center or the one that carries the largest sails.

On a three-masted ship like the famous clipper ships that raced across oceans in the 1800s, the mainmast stood in the middle, rising high above the deck. It held the mainsail, which caught more wind than any other sail and provided most of the ship's power. Sailors would climb the mainmast to adjust the sails or spot land on the horizon, a job that required courage and balance.

The mainmast was so important that damaging it in a storm could cripple a ship's ability to sail. When people talk about being tied to the mast like Odysseus (who had himself lashed to his ship's mast to resist the Sirens' song), they're usually referring to the mainmast. Even on smaller sailboats today, the mainmast remains the primary source of sailing power, though modern masts are often made of aluminum or other materials instead of wood.