The Zugot is a reference to the period bridging between the students of the Anshe Knesset Hagedolah and the Tanaim, as well as to the Sages that led these generations. This era lasted for roughly 208 years between 3560 and 3768. Each generation was led by a pair of two Sages, hence the name 'Zugot', literally 'the Pairs'. In total there were five sets of Zugot each having one Sage who served as Nasi and head of the Sanhedrin and the other as Av Beis Din.
3rd Set of Zugot Shimon ben Shetach and Yehuda-ben-Tabai
5th Set of Zugot Originally was Hillel and Menachem. When Menachem left Shami was appointed in his place.