a universal parallel ruler is disclosed . a desired angle of rotation of a preset member relative to a base line is selected . this angle of rotation is introduced into the ruler via an input station mounted on the ruler . the desired angle of rotation of the preset member is displayed on a display and an input signal representative of this desired angle of rotation is input to a comparator . the preset member is rotated . a detector detects the actual angular position of the preset member relative to the base line and it introduces another input signal to the comparator representative of the actual position of the preset member relative to the base line . when the actual angular position of the preset member equals the desired angle of rotation , of the preset member that is , when the two input signals to the comparator are approximately equal , a coincidence signal is generated which may light a lamp , indicative of this coincidence , and may activate a braking mechanism which will function to apply a braking force to the preset member , thereby preventing further angular rotation of the preset member and preventing the actual angular position of the preset member from exceeding the desired angle of rotation of the preset member input via the input station . the scale is then rotated to the new position of the preset member and locked in its new position . alternatively , a desired angle is memorized in the input station and input to a comparator . the actual position of the scale is input to the comparator . upon coincidence , an output from the comparator brakes the scale rotation .