Before 1997, the idea of one country using two currencies was almost unimaginable. Yet behind Hong Kong's Bauhinia coins lies a little-known story of careful planning and secrecy.

Before the handover, Hong Kong's coins featured the portraits of British monarchs, from the King to the Queen. But officials faced a major challenge: how could a completely new set of coins be circulating by July 1, 1997, the day sovereignty was returned to China?

The answer was to start much earlier. In 1993, Bauhinia coins quietly entered circulation, giving Hong Kong several years to gradually replace coins bearing the Queen's portrait instead of attempting an overnight change.

But designing the new coins was no simple task. The design had to be acceptable to both China and Britain while remaining politically neutral.

The secret mission fell to Joseph Yam, then Chief Executive of the Exchange Fund, who would later become the first Chief Executive of the Hong Kong Monetary Authority. To keep the project confidential, he used the pretext of designing a logo for the soon-to-be-established Monetary Authority to obtain photographs of the Bauhinia flower from the Government Information Services. He then commissioned calligraphy for the Chinese characters "Hong Kong" and personally photocopied and assembled the first draft of the coin design.

The Bauhinia has appeared on Hong Kong's coins ever since, becoming one of the city's most recognisable symbols.

Today, as digital payments become increasingly common, physical coins are easy to overlook. But each Bauhinia coin is more than spare change. It is a reminder of Hong Kong's transition, and of the careful planning behind one of the city's most significant moments in history.