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Calipers have a limited measurement range, usually to 25 mm/1 in , while micrometers are more accurate. The caliper, for one thing, will typically measure 0-50 mm/6 in., but can span to 2,000 mm/80 in., depending on the length of the scale. External measurements are made by closing the jaws over the piece to be measured, and internal measurements are made by opening up the inside diameter contacts. There are three different caliper types. The original design of the vernier caliper is the most rugged. Graduated much like a micrometer, it requires the alignment of an etched scale on the vernier plate with an equally spaced scale running the length of the tool's handle. Skillful tool alignment and interpretation is necessary to achieve the stated accuracy. The dial caliper replaces the vernier scale with a dial indicator. The indicator is fixed to the moveable jaw and engaged with a toothed rack on the body of the unit. The dial, which is typically balanced (i.e., can move in either plus or minus directions from zero), may be graduated in either inch or metric units. The dial caliper is a dual-purpose tool for making either direct or comparative measurements. Another useful feature of the dial caliper are jaws that slide past each other to allow contact points or depth rod extensions to fit into narrow openings for small ID measurements. In the last 20 years, numerous electronic features have been developed to make digital calipers easier to use without adding substantial cost. These include easy switching between inch and metric units, tolerance indications, digital output to electronic data collection systems, zero setting anywhere along the caliper's range, and retention of the zero setting even when the caliper is turned off. With no moving parts in the readout, digital calipers are exceptionally durable.