A blackboard is a large, dark surface mounted on a wall where teachers write with chalk so everyone in a classroom can see the same information at once. The board is usually black or dark green, and the white or colored chalk shows up clearly against it. Teachers use blackboards to work through math problems, diagram sentences, draw maps, or list homework assignments.

Before blackboards became common in the early 1800s, teachers could only show written work to one student at a time. The blackboard revolutionized education by letting a teacher explain an idea to thirty students simultaneously. A teacher might write a challenging word on the blackboard while discussing it, or sketch a quick drawing to illustrate a concept.

The main drawback is chalk dust, which can make people cough, and erasing the board with a felt eraser can create clouds of white powder. Many classrooms today use whiteboards with markers instead, but the word blackboard persists. When someone says “let's put this on the blackboard,” they mean making an idea visible and public so everyone can examine it together, even if the actual board is white, green, or digital.