The number of a clock (in a hierarchical synchronization distribution system) that defines the quality of the clock by specifying parameters such as frequency accuracy, pull-in range, and noise performance. The formal specifications are given in T1.101-1999 and T1.105.09-1997. Note: Only stratum 1 clocks may operate independently; other clocks are slaved directly or indirectly to a stratum 1 clock. [T1.X1]