As well as the pneumatic doors Moholy-Nagy also created a poster promoting escalators on the Tube. London Transport must have been particularly proud of their new pneumatatic doors though and the poster explained how they worked "The doors which are edged with rubber are kept closed by air-engine arms one for each door. The arms engage in slots in the doors which open when the arms swing round. The doors are held open by the arms. The signal to start cannot be given while the doors are open".