A military uniform is a standardised dress worn by members of the armed forces and paramilitaries of various nations. Military dress and styles have gone through significant changes over the centuries, from colourful and elaborate, ornamented clothing until the 19th century, to utilitarian camouflage uniforms for field and battle purposes from World War I (1914–1918) on. Military uniforms in the form of standardised and distinctive dress, intended for identification and display, are typically a sign of organised military forces equipped by a central authority. Military uniforms differ not only according to military units but tend to also be offered in different levels of formality in accordance with Western dress codes: full dress uniform for formal wear, mess dress uniform for semi-formal wear, service dress uniform for informal wear, and combat uniform (also called "battle/field dress") which would equal casual wear. Sometimes added to the casual wear category is physical training uniforms. A distinction should be made between uniforms and ethnic dress. If a particular people or culture favoured a distinctive dress style this could easily create the impression of uniformly dressed warriors. The issue is further complicated by the distinctive features (weapons, armour, fighting style and native dress) of particularly effective warrior classes often being copied. Thus the distinctive and colourful clothing of the Hungarian hussars became a model for hussar units all over Europe. The kilts and sporrans of Scottish Highland clans were distilled into regimental dress when the British Army started to recruit from these tribal groups. Mercenary or irregular fighters could also develop their own fashions, which set them apart from civilians, but were not really uniforms. The clothing of the German Landsknechte of the 16th century is an example of distinctive military fashion. Special units such as Zouaves developed non-standard uniforms to distinguish them from troops of the line.