 Where two of the aryl groups are bridged by a heteroatom, these triarylmethane compounds may be further categorized into acridines (nitrogen-bridged), xanthenes (oxygen-bridged), and thioxanthenes (sulfur-bridged).The amine-containing dyes are often prepared from Michler's ketone or its diethylamino analogue.In this way, the third aryl group is readily differentiated.