Concertina doors are a general name used, in the UK and Australia particularly, for a type of door that does not open outwards or inwards by way of a hinge but rather folds vertically in one or more places, giving the appearance of a concertina, hence the name. Incidentally, this type of door is referred to as ‘harmonica doors’ in the US and elsewhere, which has mostly to do with what the musical instrument is known by in those countries.
A concertina door can be divided up into any number of vertical subdivisions, depending on the width of the door and the application. But a typical interior concertina door of standard width would have 7-10 vertical sections, although they can have as few as two. These are attached by some kind of pivot so that they fold (or ‘stack’) alternately inwards and outwards, much the same as if you took a piece of paper and folded it into strips, alternating first one way then the other, until you had, well, a concertina shape! Concertina doors are usually fixed at one end, and this fixed vertical strip is actually the only one which rotates out from the doorframe as a regular door would. The vertical section at the other end is the one which comes into contact with the other side of the doorframe and typically has some kind of catch or lock which allows it to be closed fully and stay put.
There also has to be some kind of mechanism for the doors to be able to slide easily open or closed – if you think about it, each section actually needs to slide freely left or right, as the ‘concertina’ is ‘squeezed’ open or closed. This part of the door can be the most crucial, as a cheap, poor mechanism will not open or close properly, and the door will constantly stick when used. Though this may seem trivial, it can become an intense irritation whereas a smoothly sliding mechanism will be a pleasure to use. If you see concertina doors for sale cheaply, be aware that this may be exactly where they have cut costs, and you may end up regretting having tried to save a few pennies.