Mechanical and structural tube have a lot in common. Both types can be square, round, or rectangular. Both mechanical and structural tube are measured by inside dimensions (I.D.), outside dimensions (O.D.), and gauge (wall thickness). Both can be made of stainless steel, carbon steel, galvanized steel, or other steel alloys … as long as the material is strong enough to stand up to stressful structural use. Both types can generally be fabricated after production including cutting, punching, stamping, welding, and other secondary services.