A jamb is the vertical side piece that forms the frame of a doorway or window. When you walk through a door, the jambs are the flat surfaces on the left and right that the door closes against. Run your hand along the side of any doorway and you're touching the door jamb.

Carpenters carefully measure and install jambs to make sure doors hang straight and close properly. If the jambs aren't perfectly vertical, the door might swing open by itself or not latch correctly.

You might also hear about window jambs, which work the same way: they're the vertical sides of the window frame. When you lean against a doorway while talking to someone, you're leaning on the jamb. And if you've ever gotten your finger caught in a closing door, you probably remember the painful meeting between door and jamb.