At present there are two types of international exhibition: World Expos (formally known as International Registered Exhibitions) and Specialised Expos (formally known as International Recognised Exhibitions). World Expos, previously known as universal expositions, are the biggest category events. At World Expos, participants generally build their own pavilions. They are therefore the most extravagant and most expensive expos. Their duration may be between six weeks and six months. Since 1995, the interval between two World Expos has been at least five years. World Expo 2015 was held in Milan, Italy, from 1 May to 31 October 2015.