An arch is a curved vertical structure spanning an open space underneath it. Arch can either support the load above it or perform a purely decorative role. The arch dates back to fourth millennium BC, but became popular only after its adoption by the Romans in the 4th century BC.
Gateway Arch
A complex funicular model (Church of Colònia Güell by Gaudi, 19th century)
Arcades of Pont du Gard (Roman)
Separating arches in the St. Zeno church [de]
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as biological organisms, minerals and chemicals. Abstract structures include data structures in computer science and musical form. Types of structure include a hierarchy, a network featuring many-to-many links, or a lattice featuring connections between components that are neighbors in space.
A traditional Sami food storage structure
Gothic quadripartite cross-ribbed vaults of the Saint-Séverin church in Paris
The spiral arrangement of small bright yellow florets that make up the flower head of a sunflower is an example of the highly ordered structure that characterizes all organisms.
![Separating arches in the St. Zeno church [de]](https://upload.wikimedia.org/wikipedia/commons/thumb/b/bf/REI_St_Zeno_23.jpg/401px-REI_St_Zeno_23.jpg)
