The two general categories of graduation are linear graduation and curved graduation. Linear graduation is used on instruments that are straight in shape. A common example is a ruler that includes spaced out inches or millimeters, which measure linear distances. Measurements can also be non-linear, such as when they are using logarithmic scales or transcendental scales. Finally, volumetric graduations fall under this broad category and are utilized to measure liquids. Curved graduations typically are found on the limb of an instrument which can be a circular arc or offshoot. This type of graduation divides a certain space into smaller angular measurements. A common example is a clock, which divides down into equally spaced minutes or seconds.