The may contain a writing surface such as beeswax on which one may take notes with a pointed stylus. Or they may have a carved or recessed painted surface, often including one or more icons. A triptych is similar, except that it has 3 panels. Usually the two side panels meet in the center of the large panel when closed. These were sometimes placed on altars or in shrines.
Small ones are easily portable.
These are commonly made of wood. The surface to be painted is usually coated with gesso befor color is applied. I have also seen small metal diptichs and triptichs which may be worn as lockets. Larger ones are sometimes associated with reliquaries. Icons by Gordana Babic, I.P. Verlgsgesellschaft.