The I-beam derives its name from the shape of its profile’s cross section. If you cut an I-beam level’s profile in half, it will look like the letter I.
The horizontal elements of the “I” are known as flanges, while the vertical element is termed “web”. The web resists shear forces, while the flanges resist most of the bending moment experienced by the beam. The I-shaped section is a very efficient form for carrying both bending and shear loads in the plane of the web, yet it has a relatively low capacity in the transverse direction and is inefficient in carrying torsion which is very important for keeping a level straight and accurate.