In 1187, Alfonso negotiated with Frederick I, Holy Roman Emperor, who was seeking to marry his son Conrad to Alfonso's eldest child and heir, Berengaria. In April 1188, they agreed on a treaty in Seligenstadt, which made clear that she was the heir of Castile after any sons of Alfonso and that Conrad would only co-rule as her spouse. That became relevant in her ultimate succession to the throne even though the marriage to Conrad was never consummated and later annulled. The treaty also documented traditional rights and obligations between the sovereign and the nobles in Castile. In July 1188, Alfonso convened his court in Carrión de los Condes to allow the nobles to review and ratify the treaty. At that court, Alfonso knighted both Conrad and Alfonso IX of León, who would ultimately marry Berengaria. The younger Alfonso had come to seek the support and acknowledgement of his ascent to the throne of León from his older cousin. The elder Alfonso granted that in exchange for acknowledgement that the king of Castile was overlord of the king of León.