In the Registered Cossack Army of the Polish–Lithuanian Commonwealth in the 16th and 17th centuries, a "polkovnyk" commanded a regiment (. "polk"), a Cossack military unit. After the reform of the Cossack army by hetman Mykhailo Doroshenko in the 1620s there were six Cossack regiments, each comprising one thousand Cossacks. "Polkovnyk"s were elected by the Cossack Council (, "rada") subject to the approval of the Polish government. A "polkovnyk" obtained a salary for his service, and enjoyed considerable privileges. After the Sejm of the Polish–Lithuanian Commonwealth adopted the "Ordination" of 1638, only noblemen ("szlachta") were allowed to become "polkovnyk"s.