It looks to me like a coronet and might symbolise her princess rank.
--Previous Message-- : It looks like a small crown without arches. : But I have no information about its origin : or its symbolism. : : : Caption: Princess Anne, The Princess Royal : leaves Saint George's Chapel in Windsor : Castle after attending the Easter Mattins : Service on April 8, 2012 in Windsor, United : Kingdom. (April 7, 2012 - Source: WPA : Pool/Getty Images Europe) : : :