This document references 4 sources about tall structures - the Petronas Twin Towers in Kuala Lumpur, Malaysia and the Leaning Tower of Pisa. The sources include an article from 1969 about the Petronas Towers, facts about the Leaning Tower of Pisa from an unknown date, information from an unknown date about the Leaning Tower as one of the Seven Wonders of the Medieval World, and a 2013 article about the history and construction of the Petronas Twin Towers.