A union may be effected in a number of forms, broadly categorized as:
In an incorporating union a new state is created, the former states being entirely dissolved into the new state (albeit that some aspects may be preserved; see below "Preservation of interests").
Incorporating unions have been present throughout much of History such as the Acts of Union, 1707 between the Kingdom of Scotland and the Kingdom of England creating Great Britain, in 1910 the colonies of the Cape of Good Hope, Natal, Orange River Colony, and Transvaal were incorporated into the Union of South Africa, between the years of 1037 to 1479 Spain was in the process of Incorporating the Crown of Castile, Aragon, and Navarre into the Kingdom of Spain, though the process wasn't completed until 1716 (Aragon) and 1833 (Navarre), the Acts of Union 1800 united the Kingdom of Ireland and the Kingdom of Great Britain into the United Kingdom, in 1990 the People's Democratic Republic of Yemen united with Yemen Arab Republic (North Yemen) to form the Republic of Yemen, and in 1783 the Articles of Confederation were signed by each of the Thirteen Colonies, uniting them into the United States of America.
Nevertheless, a full incorporating union may preserve the laws and institutions of the former states, as happened in the creating of the United Kingdom. This may be simply a matter of practice or to comply with a guarantee given in the terms of the union. For example:
In an incorporating annexation a state or states is united to and dissolved in an existing state, whose legal existence continues.