The well-known poem Nostalgia, penned by the late writer Yu Guangzhong 50 years ago, expresses the aching separation felt by a generation of homesick Chinese who lived in Taiwan, and it still resonates with people on both sides of the Strait.

It starts with the line, "When I was young, nostalgia was a tiny stamp" to symbolize the distance between him and his mother.

Yu grew up on the Chinese mainland and moved to Taiwan in 1950 at a young age. He couldn't return till decades later.

As the poem says, writing letters was a way for separated families to express their longing and ease their emotional pain. In recent years, museums and archives have started collecting and preserving these letters to document the stories of individual families.

Waves of people from Fujian, the closest mainland province to Taiwan, have migrated to Taiwan throughout history, leaving a paper trail of letters, photos and tickets about their travel and resettlement.

Shen Fushan, deputy director of Dongshan County Archives in Zhangzhou, in Fujian's south, said about 400 letters have been collected in the county since 2018, with many written by veterans in Taiwan.

The coastal county of Dongshan has had a long history of interaction with Taiwan. In 1950, when Chinese Kuomintang troops retreated to Taiwan they passed through the county. Thousands of able-bodied men were captured, forced to join the Kuomintang troops and taken to Taiwan. Most of the men who left wives and children behind in Dongshan wrote letters to them.

"Many families kept these letters and behind each one there is a touching cross-Strait family story," Shen said.

The 54-year-old remembers that when he was a child, his uncle, who was taken to Taiwan by the troops, would write to them.

From 1950 to the end of the 1980s, when communication was largely prohibited between the two sides, these letters had to be sent to a third destination, usually a Southeast Asian country, first. Acquaintances were entrusted to change the details written on the envelopes or put the letter in another envelope to hide the sender's information, before sending the letter to the mainland, Shen said.

"It could take up to six months for a letter to reach (the mainland), and some never arrived," he said, "These letters were particularly precious especially when communication across the Strait was prohibited."

He said the contents of the letters usually informed the family that they were alive and in Taiwan, and that they missed them. "Because at that time the two sides were still hostile to each other, these letters mostly avoided discussing too many personal details," Shen said.