What was William the Conqueror's real name?

His real name in French was Guillaume de Normandie. He was never called William during his lifetime as the name William did not exist at the time. His "English" name was Wilgelm (Latin Willelmus - seen shortened to Willelm). Evidence for this is found in the famous Bayeux Tapestry. The exact pronunciation of Wilgelm, this alternative name for William, is debated. However, in later centuries when the name Wilgelm is succeeded by William, historians refer to Wilgelm as William. Thus, the phrase William the Conqueror is born. In France, he was William, Duke of Normandy.