The Metropolitan Police first introduced blue police boxes (in London) in 1928 – with the now best-known design (by Gilbert MacKenzie Trench) following in 1929. Initially made of wood, the Trench design became largely concrete, which was more durable, though the doors stayed as wood (teak). Inside was for officers (for reading or completing reports, meal breaks, or temporarily holding prisoners for transport) and functional – a stool, table, incident book, brush and duster, fire extinguisher, first aid kit and a small electric heater. There was also a phone linked directly to the local police station, which could be used by officers or the public. The exterior had a light at the top; when flashing, police officers had to contact the station. The boxes stayed in use until the early 1970s, when personal radios, the 999 emergency number and public access to telephones conspired to make them somewhat redundant.