As suggested for example by Carathéodory and used for example by Callen, the favoured instance of an adiabatic wall is that of a Dewar flask. A Dewar flask has rigid walls. Nevertheless, Carathéodory requires that his adiabatic walls shall be imagined to be flexible, and that the pressures on these flexible walls be adjusted and controlled externally so that the walls are not deformed, unless a process is undertaken in which work is transferred across the walls. The work considered by Carathéodory is pressure-volume work. Another text considers asbestos and fiberglass as good examples of materials that constitute a practicable adiabatic wall.