In it's most basic form, a diptych is any object with two flat plates usually attached at a hinge. In ancient times a diptych was also a type of icon whereby two panels are joined together with a hinge, so that they may fold together for protection when travelling, and then be unfolded for veneration when one's destination has been reached. Today a diptych can also loosely refer to a work made up of two matching or co-ordinating parts and is used by artists in panel paintings and photography for example.
Gerlach Flicke; Henry Strangwish (Strangways)
by Gerlach Flicke
Queen Mary II; King William III
by Wallerant Vaillant, after Unknown artist
Anne, Princess Royal and Princess of Orange; William Charles Henry Friso, Prince of Orange
after Philip Mercier, and after Philip van Dyk
1734 or after