(Reuters) - A war of words between Donald Trump and the parents of a Muslim U.S. soldier killed in Iraq in 2004 is dominating the election campaign.  Khizr and Ghazala Khan were criticized by the Republican presidential candidate after they appeared at the Democratic National Convention last Thursday and shared the story of their son, U.S. Army Captain Humayun Khan. The Khans are what is known in the United States as a “Gold Star Family,” meaning they have lost an immediate relative in a military action. The gold star refers to the centerpiece of flags given to these families. On Monday, more than 20 Gold Star Families wrote to Trump condemning his treatment of the couple, saying he was “cheapening the sacrifice” of soldiers who have died and demanding that he apologize to the Khans.   The following are some facts about Gold Star Families: * What are Gold Star Families?  Gold Star Families have lost a member of their immediate family in a war or other conflict while serving with the U.S. military. The military awards each family an official flag bearing a gold star or a lapel pin with a gold star on a purple background.  Called a service flag, families with relatives on active duty display a flag with a blue star.   * History of Gold Star Families Grace Seibold, whose son was killed in Europe in 1918 during World War One, founded a group that came to be known as American Gold Star Mothers for women who lost sons in the war. In the same year, President Woodrow Wilson approved a suggestion to let U.S. women wear a gold gilt star on a traditional black mourning arm band signifying their loss. In 1936, the United States began observing Gold Star Mothers Day on the last Sunday of September. An organization called The Gold Star Wives was formed before the end of World War Two, and the Gold Star Lapel Button was created in 1947. The term Gold Star Families was widely used after World War Two. * Different Service Flags A blue star signifies a relative serving in the military while a gold star means a family member has died in service. During World War One, relatives of active duty service members flew service flags bearing a blue star. As the gold star tradition took hold, families covered the blue star with a gold one to symbolize their loss. These flags were often hung in windows.  * How many Gold Star Families are there? The National Gold Star Family Registry notes on its website that 472,045 fallen service members have been registered by relatives. here * Which families wrote to Trump?  Monday’s letter to Trump was signed by relatives of U.S. service members killed in Iraq, Afghanistan, Vietnam and Korea.  