Rein n.
 1. The strap of a bridle, fastened to the curb or snaffle on each side, by which the rider or driver governs the horse.
    This knight laid hold upon his reyne.   --Chaucer.
 2. Hence, an instrument or means of curbing, restraining, or governing; government; restraint. “Let their eyes rove without rein.”
 To give rein, To give the rein to, to give license to; to leave withouut restrain.
 To take the reins, to take the guidance or government; to assume control.