A tune written more than two decades ago and based on a poem nearly a century old has become a standard song that helps bond the people of the Chinese mainland and the people of Macao.

In 1997, Chinese musician Li Haiying was invited to compose a song for a documentary being produced by Chinese Central Television about Macao's return to the motherland in 1999. With the invitation from Beijing came a one-page fax quoting an old poem, Song of Seven Sons, written by Chinese patriotic poet, Wen Yiduo, during the 1920s.

"The poem is short, but it touched my heart when I first read it," Li recalled.

Song of Seven Sons is a first-person narrative composed of seven short sections. The poet described seven regions of China occupied by foreign colonists as seven siblings, driven apart from their mother.

The seven sons lamented their separation and longed to be reunited. Macao, colonized by the Portuguese, was one of the sons.

Li, who lives in Guangzhou, went to a bookstore to find something to give him broader insight into the poem, but he found nothing.

With only simple lines to serve as his inspiration, Li began composing while sitting at the piano. He said he read the lines over and over until he could hear the music behind the words.

"From the vivid narrative, I imagined a child, like an orphan, expressing his melancholy, longing to be reunited with his mother, in simple and clear expressions," the composer said.

"The wind whips, powerful waves crash on shore. Macao has endured a hundred years of pounding tides." The images played continually through Li's mind.

"When I put my hands on the keyboard, my inspiration followed the lyrics, and chorus melody and verse melody naturally flowed from my fingers." Li wrote the song in half an hour.

Recording the song required an extensive, cross-boundary production. Producers invited a 7-year-old girl from Macao to record vocals at a studio in Guangzhou. Other elements included an orchestra, a children's choir and a mixed choir in Beijing.

"With lyrics written in 1925, a melody composed in 1997, a song sung by people of different ages and places, it took an impressive amount of cooperation stretching the limits of time and space," Li said.

The song became a big hit in China after the documentary was broadcast. Soon after, it was designated as the theme song for Macao's return to the motherland in 1999.

Li was named "Special Contributor to Macao" by the city's government in 2009. Only one other person who was not a Macao resident has been granted the honor.

"The song is so popular in Macao that almost everyone can sing its melody, though some cannot remember all lyrics clearly," said Ally Li, who works in Macao's cultural sector.

In 1999, Ally Li, who was a secondary school student, fell in love with the song. "The simple but touching melody and the lyrics blend into a harmonious whole. It can easily pull listeners' heartstrings."

A video related to the song recently went viral on Weibo, featuring 20 Macao primary school students and a group of mainland pupils singing together.

When the choir sang, "Mom, I have missed you for 300 years, please call me by my birth name: Aomen (Macao in Chinese)", the audience in the video spontaneously stood up and joined the singing.