The Senior was tasked with defense of borders, the right to have troops in provinces of other Dukes, carrying out the foreign policy, supervision over the clergy (including the right to nominate bishops and archbishops), and minting the currency.

He was followed in the Seniorate by the second eldest Mieszko III the Old, while Masovia and the Kuyavian lands passed to Bolesław's IV minor son Leszek.

Though upon the death of Bolesław IV the Curly he had received the Duchy of Sandomierz, in 1177 he took the occasion of an uprising by Lesser Polish nobles (magnates) and assumed the rule as High Duke from his elder brother Mieszko III.

A long-term struggle between the brothers followed, whereby Mieszko III was able to incorporate the northwestern lands of Gniezno and Kalisz into his Duchy of Greater Poland.

In the long-term struggle Leszek I was killed in 1227 and the Pomerelian lands got lost, when Duke Swietopelk II of Gdańsk declared himself independent.