International exhibitions in the 19th Century were categorized^[*] as follows:

1. Registered Exhibitions are the major events and their contents may encompass many areas of progress or innovation. Their duration can be from six weeks to six months. Individual events are separated by an interval of five years. A proposal to the BIE for selection as host country must be lodged with the bureau five to nine years in advance of the proposed expo date.
2. Recognized Exhibitions are smaller in scope and more specific in terms of content. Having a shorter duration – between three weeks and three months – recognized exhibitions may be staged on a yearly basis, provided the proposed year does not coincide with that of a Registered Exhibition. A proposal for BIE selection as host country must be lodged with the bureau four to five years in advance of the proposed exhibition date.

[*] Source: Bureau of International Exhibitions at www.bie-paris.org