The Queen and Prince Philip have been together for decades, having married in 1947.
Although few knows what the hidden message says, the Queen has spoken candidly of her husband many times.
Celebrating their older anniversary in 1997, the monarch said of her husband: “He is someone who doesn’t take easily to compliments but he has, quite simply, been my strength and stay all these years, and I, and his whole family, and this and many other countries, owe him a debt greater than he would ever claim, or we shall ever know.”
For their 70th wedding anniversary in 2017, the Queen honoured her husband with a new title.
Prince Philip became Knight Grand Cross of the Royal victorian Order.
According to a statement from Buckingham Palace, the honour is a form of chivalry personally given by the Queen for “services to the sovereign”.