Tian Xia Wei Gong ("what's under the heaven is for all") is one of Sun Yat-sen's most famous calligraphic works. [Photo provided to China Daily]

The pledge was made in three copies, with the couple keeping two. But the copies in their possession got lost when Soong had to leave Shanghai before it was occupied by the invading Japanese troops in 1937.

Four decades later, a said copy of the document was found in the collection of National Museum of China, which was then called the Museum of Chinese Revolutionary History. It was presented to Soong who confirmed its authenticity and signed her name in endorsement. She also wrote, "It is the real one."

The copy, together with Soong's written authentication on a separate piece of paper, is being shown at an exhibition commemorating the 150th anniversary of Sun's birth, at the National Museum of China in Beijing. It is displayed with many photos of the couple, such as their famous wedding photo taken by a Tokyo studio and their last photo together at a Kobe hotel on Nov 24, 1924.

The display is a "testimony to their bond as loving and revolutionary partners" and it realizes Soong's long-cherished wish, says Li Liang, a curator at the museum.

The exhibition, titled The Whole World as One Community, shows more than 300 documents, photos and relics from the museum's collection that reviews Sun's contribution to the revolution.

Lyu Zhangshen, the museum director, says 72 of the exhibits, including the vow, are on show for the first time.

Sun was an avid calligrapher who passionately expressed his thoughts with strokes of ink.

After the Republic of China (1912-49) was founded, he became its first president but suffered many setbacks, with his life often under risk. He wrote calligraphy works to encourage both himself and his supporters.