Quater'nion (τετράδιον, a body of fl.bur). "A quaternion of soldiers" (Ac 12:4) was a detachment of four men, which was the usual number of a Roman night-watch (Veget. De Re de Milit. iii, 8; Philo, 2 In Flacc. p. 98; Polyb. 6:33, 37). SEE SOLDIER. Peter, therefore, was guarded by four soldiers, two within the prison, probably attached to his person, and two outside the doors; and, as the watch was usually changed every three hours, it was necessary that the "four quaternions" mentioned in the text should be appointed for the purpose. SEE PRISON. Or one set of sentinels may have been posted at the door of the cell (which was probably thought to be so secure as not to require a guard within), and another at the outer or street gate (Walch, De Vinclis Petri, in his Dissert. ad loc.). SEE PETER.