A folding camera is a camera with a bellows that can be folded to hold less size. All bellows cameras won't be called foldings: the monorail view cameras have a bellows, but they cannot be folded to a more compact size.
We usually distinguish between three architectures:
- the folding bed cameras (see Agfa PB 20 for a picture)
- the strut folding cameras (like the Plaubel Makina)
- the barn door folding cameras (like the Voigtländer Vitessa)
There were foldings in all kinds of format, from 18x24mm to at least 18x24cm.