On May 9, 1927, the Duke of York took the exquisite and rather delicate looking key and unlocked the heavy wooden doors that looked out onto a bare paddock, opening, in some respects, a brand new nation.

If you're wondering whether the key might still actually unlock those doors, apparently the locks have been changed since then.

The key is a centrepiece in a new exhibition at the Museum of Australian Democracy, The Opening Day: 9 May, 1927, marking Old Parliament House's 90th year.

The key usually resides in a cabinet in the office of the Speaker of the House of Representatives. It has been loaned to the exhibition by current Speaker Tony Smith for the first six weeks of the exhibition, which will continue all year.

Director of the Museum of Democracy Daryl Karp suggested the museum might be a good place for the key to find its permanent home, light-heartedly bantering with Mr Smith, "talking politics in the most appropriate of places," she said.

The key itself has a rather mysterious story. Apparently three keys were to be made in case one was misplaced before the day, but only one has ever been seen. It was designed by Edwin Henderson, a senior architect with the then Department of Works and Railways, who went on to design Manuka Pool and Ainslie Primary School. It was made by Hardy Bros Jewellers, in either Sydney or Melbourne, depending on which paper you read.

Stephanie Pfennigworth, the curator of the exhibition, said it was nice to have the key back where it all started.

"It was actually used, I believe, with the Duke turning it in the latch, so it wasn't just a ceremonial thing."

The Opening Day tells the stories of the people who witnessed a historic chapter in Australia's democratic story. Through the photographs, recollections and memorabilia of onlookers at the opening ceremony, the exhibition reveals rich stories about the life and times of the nation.

"The opening of Parliament House represented nationhood, identity and ideals," Ms Karp said.

"On its 90th anniversary it is important we commemorate this important chapter in Australia's democratic history."

The exhibition is spread throughout the halls of Old Parliament House. In King's Hall, there's a collection of memorabilia, clothes people wore to the ceremony, official programs and photographs. And then you're taken on a journey of sorts, through the corridors of power, following the planning of the building of the house to the opening day ceremonies and the spectacle that followed.

The Opening Day: 9 May 1927, at the Museum of Australian Democracy, Old Parliament House, will be open until May, 2018. The gold key used by the Duke of York to open the building is on display in King's Hall for six weeks only.