Rectangular hollow section. This is a common profile of structural steel tubing used for columns, posts, and framing where bending forces are expected in multiple directions. It offers a high strength-to-weight ratio and a flat surface that makes welding and bolting easier than with round pipes.
Heard on the job
We need to order some 100 by 50 RHS for the new canopy posts.
Insider tip
RHS is usually specified by its outside dimensions and wall thickness, like 150x100x6.