Folding-growing. These are the so-called doors, which are popularly called «accordion». Unlike compartment -type doors, «books» do not require a connector for the wings, they just add up. In general, their main advantage can be called compactness. Of course, in a closed form they will occupy a certain space, but it should be borne in mind that such doors have very small thicknesses, therefore, the space will be occupied only slightly;