This combination photo shows tourists visiting the Colina da Guia in Macao, south China on Oct. 18, 1999 (top, photo by Wang Yuguo) and on Dec. 17, 2014 (bottom, photo by Lui Siu Wai), respectively. 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)