Over the years, both the width and thickness (strength) of mainsprings has been measured in different ways on various gauges, ranging from brass or steel plates with notches of various sizes cut in them for measuring the width, and narrowly tapered slots for measuring the thickness or strength. In later times micrometers were used, which more accurately measured in hundredths of a millimetre. The correct thickness of a spring also had to relate to the diameter of the barrel into which it fitted, to give the correct number of turns per day to the barrel.