Until recent times, schools have been characterised by the physical presence of teachers and students together. Usually, a building is used for instruction, and teaching materials such as books or blackboards are often in evidence. In the 20th century, alternatives to what may be called “bricks-and-mortar” schools emerged. These were forms of distance education, where children could learn without attending classes on a regular basis. The technologies used included mail, for correspondence schools, and the 20th century technologies of radio and television.