This combination photo shows mainland residents passing the border control section at a Customs building in Zhuhai, south China's Guangdong Province, before traveling to Macao, south China on Oct. 2, 1999 (top, photo by Zhang Yanhui) and a traveler going through an e-channel from a Customs facility in Macao to Zhuhai, south China's Guangdong Province on Dec. 18, 2014 (bottom, photo by Qin Qing). On Dec. 20, Macao will celebrate the 20th anniversary of its return to motherland. Over the past two decades, the special administrative region has made great strides in economic development and achieved prosperity and stability under the "one country, two systems" principle. Since Dec. 20, 1999, the historic day for Macao, what has changed? And what remains unchanged? Our selection of photos present comparative illustrations of the south China city's past and present. (Xinhua)