The duchy was named after the town of Geldern (Gelder) in present-day Germany. Though the present province of Gelderland (English also Guelders) in the Netherlands occupies most of the area, the former duchy also comprised parts of the present Dutch province of Limburg as well as those territories in the present-day German state of North Rhine-Westphalia that were acquired by Prussia in 1713.
Four parts of the duchy had their own centres, as they were separated by rivers:
spatially separated from the Lower Quarters (Gelderland):
The county emerged about 1096, when Gerard III of Wassenberg was first documented as "Count of Guelders". It was then located on the territory of Lower Lorraine, in the area of Geldern and Roermond, with its main stronghold at Montfort (built 1260). Count Gerard's son Gerard II in 1127 acquired the County of Zutphen in northern Hamaland by marriage. In the 12th and 13th century, Guelders quickly expanded downstream along the sides of the Maas, Rhine, and IJssel rivers and even claimed the succession in the Duchy of Limburg, until it lost the 1288 Battle of Worringen against Berg and Brabant.
Guelders was often at war with its neighbours, not only with Brabant, but also with the County of Holland and the Bishopric of Utrecht. However, its territory grew not only because of its success in warfare, but also because it thrived in times of peace. For example, the larger part of the Veluwe and the city of Nijmegen were given as collateral to Guelders by their cash-strapped rulers. On separate occasions, in return for loans from the treasury of Guelders, the bishop of Utrecht granted the taxation and administration of the Veluwe, and William II ― Count of both Holland and Zeeland, and who was elected anti-king of the Holy Roman Empire (1248–1256) ― similarly granted the same rights over Nijmegen; as neither ruler proved able to repay their debts, these lands became integral parts of Guelders.