Their ancestors were "royal friends by correspondence" but Britain's Queen Elizabeth II and Thailand's King Bhumibol developed a face-to-face bond during their lifetimes.
The late King Bhumibol Adulyadej, also known as Rama IX, was the second longest reigning monarch in world history until his death in 2016 at age 88 -- serving on the throne for seven decades and 126 days.
It was a record the late Queen Elizabeth surpassed in June before she died aged 96 on Thursday at her Scottish Highland retreat.
The Thai king had a head start, beginning his reign in 1946, while the British queen ascended the throne six years later.
But neither reached the late French King Louis XIV's record: 72 years and 110 days, which ended in 1715.
- Royal friendship -
The pair forged a friendship over the decades -- smiling warmly and chatting merrily as they interacted and welcomed each other on state visits, according to archival footage.
Britain was the first stop on the Western-educated King Rama IX's six-month European tour in 1960.
Queen Elizabeth and her husband, the late Prince Philip, were on hand to greet the Thai monarch and his wife Queen Sirikit at Victoria Station in London.
Platform two had been jazzed up for the grand occasion -- with long curtains with blue and gold tassels and giant vases of flowers including lilies and carnations.
But a BBC presenter was unimpressed and lamented the UK was unable to provide a more "glamorous" portal of arrival, noting Thailand's exotic golden pagodas, teak forests and elephants.
After a royal guard inspection, the two heads of state shared a horse-drawn carriage ride to Buckingham Palace, as well-wishers waved flags in the streets.
In February 1972, Queen Elizabeth visited Thailand for the first time, accompanied by Prince Philip and daughter Princess Anne.
The queen caused somewhat of a stir wearing a blue polka dot dress that was slit to the waist on both sides, revealing a white undercoat underneath.
Accompanied by the Thai king, Elizabeth rode in a vintage yellow Daimler, which was soon weighed down with flowers and gifts from members of the public.
She attended a dinner reception at Bangkok's Grand Palace and had a walk about Chulalongkorn University
After receiving a key to Bangkok, the British royals also visited Ayuthaya -- the former capital of what was once called Siam, now Thailand -- where they toured the Bang Pa-In Palace.
They also ventured north to Chiang Mai and the Queen inspected handicrafts and orchids as traditional music rang out.
"Long live the queen," read a large welcome banner at the venue.
Her second trip to the Thai kingdom coincided with the year of King Bhumibol's golden jubilee.
During the October 1996 five-day visit, Queen Elizabeth admired a procession of royal barges on the Chao Praya River in Bangkok -- piloted by scores of rowers in colourful traditional costumes and golden helmets.
Later at a banquet reception -- wearing a white dress with a saffron sash -- the Queen toasted the friendship between both monarchies over several generations.
She noted Queen Victoria, her great-great-grandmother, had been pen pals with Thailand's King Rama IV, Mongkut, and that bond "has been carried forward to our generation".
"Over the last quarter of a century your country has become a sophisticated modern state with an increasingly confident democracy," Queen Elizabeth said.
"Your people's capacity to extend the friendliest of welcomes to visitors is undiminished."
In Thailand, the monarch is considered a semi-divine figurehead and the royal family is protected from insults and criticism by some of the harshest lese-majeste laws in the world -- with up to 15 years in jail per charge.
The Thai media has been much more restrained in its reporting on the Southeast Asian nation's royal family, compared to the British tabloid press.
While Britain goes into 10 days of national mourning, Thais wore black for a year as part of the kingdom's grieving period for the beloved King Bhumibol.