The formwork is an auxiliary structure that serves to temporarily support the weight of an arch "Arch (construction)") or vault, as well as other stonework, during the construction phase. This is usually a wooden "Lattice (engineered)") truss. This structure, once the voussoirs and the key "Key (architecture)") have been assembled, is dismantled in an operation called decimbrado.

During the process of building arches "Arch (construction)") and vaults are used to hold the voussoirs until the moment of their completion, when the key "Key (architecture)"). The interior curvature (or intrados) of an arch or a vault is also called falsework. In the construction and design of arches, their use is usually avoided due to the increased cost involved in their construction. It is generally commissioned from specialized carpenters.

In some countries in Spanish-speaking America, the term formwork is also used to designate formwork, due to the similarity of functions with these and, especially, when it comes to auxiliary structures of large concrete arches in bridges.