Tools that measure the difference between any two points can be divided up according to how accurately they do it.
A measuring tape is useful for checking spring sag.
A steel rule's capable of accurate measurement, down to a millimeter or a fraction of an inch.
If a distance needs to be measured with a high degree of accuracy, vernier calipers can be used. The sliding jaw grips the surfaces being measured - on the outside, or for an internal measurement, with these jaws on the inside. Depth can be also measured with the end of the caliper slide. Once the measurement is taken, this screw locks it in. Inches or millimeters are read here, and the fractions on this vernier. That gives this the name of vernier calipers.
Other calipers show the fractions on a dial or an electronic readout. This is a precision instrument that'll measure down to 2 hundredths of a millimeter or a thousandth of an inch.
For the highest accuracy in measuring distances, micrometers are used. Like calipers they can measure an outside dimension, inside, or depth, but a different "mike" as they're called is needed for each task.
The most common is an outside micrometer. The object to be measured is nipped very lightly between the anvil, the part that stays still, and the spindle which moves towards it on a very fine thread. The distance between them is read off the scale on the barrel, down to the finest fraction which is read off the scale that turns on the thimble.
For inside measurements, the inside micrometer works on the same principles as the outside micrometer, and so does the depth micrometer. They give a very accurate readout of even the tiniest movement of a very finely threaded spindle. They must be kept clean, especially along surfaces that do the measuring. They should read zero when fully closed, and be checked and adjusted as described in the instructions that come with them.
For measuring distances in awkward spots like the bottom of a deep cylinder, the telescopic bore gauge has spring-loaded plungers that can be unlocked with this screw so they slide out and touch the walls of the cylinder. The screw then locks them in that position, the gauge can be withdrawn, and the distance across the plungers can be measured with a micrometer or calipers to convey the diameter of the cylinder at that point.
In automotive workshops, clearances and narrow gaps often need to be measured. This is done with a set of feeler gauges. The markings on these strips show they're graded from fractions of a millimeter up to a few millimeters or fractions of an inch. They’re used by finding one that fits smoothly in the gap being measured. Sometimes the best fit must be made by using a combination. Then the measurement is the total thickness of all the gauges that fitted into the gap.
Roundness and squareness sometimes need measuring. This crankshaft can be rotated in these vee blocks. If it's bent, it will show on the dial gauge. It senses slight movement at its tip, and magnifies it into a measurable swing on the dial.
To check whether something is square, parallel, flat, or true, a straight edge is used. It's placed against the surface that needs checking.
A try square is a rectangular blade fitted at precisely 90° to a solid stock. A simple idea and simple to use.