Article: One of the most important victories of the magnates was the late 16th century right to create ordynacja's (similar to majorats), which ensured that a family which gained wealth and power could more easily preserve this. Ordynacje's of families of Radziwiłł, Zamoyski, Potocki or Lubomirski often rivalled the estates of the king and were important power bases for the magnates.

Question: What was an important power basis for the magnates?
estates of the king