What is a header in framing?

A header is a beam placed across the top of a door, window, or other opening to carry the loads from above and transfer them to the framing on each side.

When you cut an opening in a load-bearing wall, the studs that would have carried the load are removed, so a header bridges the gap and routes that weight down through the king and jack (trimmer) studs to the framing below.

Header size depends on the span of the opening and the load above it, including how many floors or roof loads bear on that wall. The same term also describes the framing member that closes off the ends of joists around an opening such as a stairwell.