Later this year, Indian President Pratibha Patil will become the first woman non-royal head of state to be hosted by Queen Elizabeth II in her 57-year reign as British monarch.
When 'commoner' Patil comes visiting Britain Oct 27-29, she will be only the fourth female head of state to be staying with the Queen on her invitation - all the other 94 have been men.
Although the Queen hosts many female heads of state for receptions - some of them non-royal, such as the Argentine president - they have come on invitations from British prime ministers.
Argentina's Cristina Fernandez de Kirchner, for instance, was a commoner hosted by the Queen for a reception during the meeting of the Group of 20 (G20) countries in April, but she - like the other summiteers - had come on an invitation issued by Prime Minister Gordon Brown.
Most of the women who have come visiting on the Queen's invitation have been the wives of heads of state, including President R Venkataraman's wife Janaki in 1990.
"This is a very interesting fact," a spokeswoman for the Queen told IANS.
"But it's not very obvious at first glance," she said, going through the list of dignitaries who have visited Britain on the invitation of the Queen since her accession to the throne in February 1952 - a list that the spokeswoman said was comprehensive and accurate.
One reason is the sheer lack of women heads of state around the world who have been either elected or appointed to the office, as against inheriting it - there have only been some 40 of them since Queen Elizabeth began her ceremonial role in 1952.
But the spokeswoman said Patil's being a woman "wouldn't matter much to the Queen, who treats all her guests equally".
The list of the Queen's distinguished official guests reveals only three other female heads of state but all are of royal stock: Queen Juliana of the Netherlands in 1972, Queen Margrethe of Denmark in 1974 and 2000, and Queen Beatrix of the Netherlands in 1982.
Amid the preponderance of male visitors the 83-year-old monarch hit it off with US first lady Michelle Obama when she came accompanying her husband during the G20 summit.
Having to deal with some of the badly behaved men around her can sometimes appear to test the patience of a royal known for her calm demeanour.
She treated Italian Prime Minister Silvio Berlusconi like an errant schoolboy when he shouted out to the US president after a G20 group photo, "Mr Obama! Mr Obama!"
And when former Australian Prime Minister Paul Keating put his arms around the Queen in 1992, a furious British media dubbed him the 'Lizard of Oz'.
In contrast, there was not a trace of the raised eyebrow when Michelle Obama greeted the Queen by putting her arms around the royal shoulders in June this year. Indeed, the Queen responded by wrapping her arms around Michelle Obama's waist in what a Palace spokesman described as a "mutual and spontaneous display of affection".
It could be an equally friendly woman-to-woman encounter when the current Indian president arrives in Windsor Castle - said to be the Queen's favourite weekend retreat - for her three-day stay in October.