Merriam-Webster Collegiate Dictionary - diptych

 noun  Etymology: Late Latin ~a, plural, from Greek, from neuter plural of ~os folded in two, from di- + ptyche fold  Date: 1622  1. a 2-leaved hinged tablet folding together to protect writing on its waxed surfaces  2. a picture or series of pictures (as an altarpiece) painted or carved on two hinged tablets  3. a work made up of two matching parts