The Yser Testament (Dutch: IJzertestament), officially the Open Letter to the King of Belgium Albert I (Dutch: Open brief aan den Koning van België Albert I), was an 11-page open letter addressed to King Albert I and published on 11 July 1917 during World War I. The letter's author, the philologist Adiel Debeuckelaere, set out a number of grievances relating to the treatment of the Flemish within the Belgian Army fighting on the Yser Front during World War I, especially concerning the perceived inequality of French and Dutch languages. It demanded that new legislation to equalise the status of the two languages be introduced after the war. The letter was the most famous action of the Frontbeweging and is considered an important moment in the history of the Flemish Movement.