A crossbar is a horizontal bar that connects two upright posts or poles. You see crossbars every day in soccer goals and football goalposts, where they form the top edge that the ball must go under to score. In bicycles, the crossbar is the horizontal tube that runs from the handlebars to the seat, giving the frame its strength and structure.

The word makes perfect sense when you break it down: it's a bar that crosses from one side to another. Crossbars appear in all kinds of structures where strength and stability matter. A playground's monkey bars have crossbars for swinging. Window frames often have crossbars dividing the glass into sections. Even old-fashioned telephone poles had crossbars holding multiple wires.

Crossbars do important work: they keep things rigid and prevent wobbling or collapse. Without the crossbar, a soccer goal would just be two lonely poles that could easily tip over. The crossbar distributes weight and pressure across the whole structure, the same way the horizontal piece in the letter H keeps the two vertical lines properly spaced.