The subject of materials and construction concerns not only the wide range of materials used but also their production and the nature of the building industry. The most common materials are timber, baked and unbaked clay, stone, slate, reeds, grass or straw, glass, concrete, iron and steel. The choice of materials and constructional system sets limits and creates opportunities for architects and builders. Some materials, such as brick, wood and stone, may be very familiar, but it needs careful observation to distinguish particular varieties of timber such as oak or ash, and the differences between the many varieties of stone such as sandstone, limestone and granite. One of the best ways to recognise these differences is to travel widely, look at the materials used, read guides on the local architecture which identify them and visit geological or local museums which may display local building materials.