Royal icing is a hard white icing, made from softly beaten egg whites, icing sugar (powdered sugar), and sometimes lemon or lime juice. It is used to decorate Christmas cakes, wedding cakes, gingerbread houses and many other cakes and biscuits. It is used either as a smooth covering or in sharp peaks. Glycerine is often added to prevent the icing from setting too hard. When placing icing on cakes, Marzipan is usually used under the royal icing in order to prevent discoloration of the icing.