A joist is a long, sturdy horizontal beam that supports a floor or ceiling. If you've ever seen a house being built, joists are those parallel wooden or steel beams that run across the frame before the floor gets put down. They're usually spaced about 16 inches apart and work together like the rungs of a ladder turned sideways, creating a strong framework that can hold up everything above them: people, furniture, pianos, bookshelves, and more.

Joists do the hidden but crucial work of distributing weight evenly across a structure. Without them, floors would sag or collapse. They span the space between walls or larger support beams, and carpenters nail or screw the floorboards directly onto them. If you've ever been in an unfinished basement or attic, you've probably seen joists running overhead.

Builders talk about floor joists under floors and ceiling joists overhead. Strong joists are what make it possible to jump around upstairs without crashing through the floor into the room below.