A tracheostomy tube assembly comprises an outer tube and an inner cannula. The machine end fitting of the outer tube has a bore of circular section interrupted by two flats. The machine end fitting of the inner cannula has an external surface shaped to fit within the fitting on the outer tube and also has two flats aligned with the flats on the outer tube fitting. Tubes of different sizes or other characteristics have flats with different orientations such that only inner cannulae suited for a particular outer tube can be inserted in the correct tube.
