A master beam[1]​ or girder is a beam used in construction to bridge the distance between two supports, serving as the main horizontal support of a structure formed by other smaller beams.[2]​ Beams often have a double "T" cross section "Section (geometry)" (composed of two horizontal load-bearing wings connected by a vertical web that stabilizes them), but they can also be box-shaped, Z-shaped, or other different forms. They are commonly used to build bridges and decks.

In traditional timber-framed structures, the term girder is specifically applied to the main beam that forms the crest of a gable roof.[3]​.

"Master beams" are beams that are characterized by supporting a structure made up of other smaller beams. Like any other type of beams, they can be made with very different materials.

In traditional architecture, they used to be made of wood, although the development of the steel industry in the middle of the century led to the use of beams first of padded iron and then of steel in constructions with increasingly larger spans.

Small-section steel girders are normally manufactured directly through a rolling process, while larger beams (one meter deep or more) are constructed by joining large flat pieces of steel (by welding, bolting, or, as in the past, bolting).[4]​.

Warren type beams replace the solid web with an open lattice between nodes. This arrangement forming a lattice "Armor (structure)") combines strength with economy of materials and can therefore be relatively light. Patented in 1848 by its designers, James Warren "James Warren (engineer)") and Willoughby Theobald Monzani, its structure consists of two straight and parallel horizontal chords located longitudinally, joined only by a series of oblique uprights, distributed to form alternately inverted equilateral triangles, ensuring that no individual strut, beam or tie is subjected to bending forces or torsional forces, and supporting only tractions or compressions. It is an improvement over the Neville lattice, which uses a distribution with isosceles triangles.

The appearance of reinforced concrete and then prestressed concrete at the end of the century and the beginning of the century, have allowed the massive use of prefabricated main beams in structures with spans of up to 40 m, advantageous due to the simplicity of their construction and their reduced maintenance requirements.