Measure Lots, Cut Once

Humans are tool users, and one of the important aspects of tool use is accurate, repeatable measurement. Prior to the 1800s, there were hundreds of systems of measurement with thousands of different units around the world, but today we're pretty much down to two: Metric and the American Customary System, often erroneously equated with England's Imperial System of Units. 

Whether working with wood, metal, or other materials, in order for parts to fit properly they need to be correctly sized. If duplicating an existing object, a piece of string can be used to transfer measurements from the original to the raw material of the copy. Beyond that, accurate measurements need to be taken and recorded.

Nearly all the measuring devices I'm going to mention here are available in both Metric and American Customary Unit versions. I'll also be focusing on dimensional measurements, not volume or weight.

Rulers

Perhaps the most common of measuring tools is the ruler, whether it is a simple wooden school ruler, triangular scale ruler, tape measure, or yard stick. Traditionally, rulers are marked with large numbered indicators divided by smaller, differently sized marks, collectively called rules. By lining these up against the object to be measured, a reasonably accurate value can be determined. A desk ruler might be marked in quarters or eighths of an inch, while a carpenter's ruler will more likely be marked in thirty-seconds or even sixty-fourths of an inch divisions.

A selection of linear measuring tools

Calipers

This type of measuring device comes in two main categories, dial (or digital) calipers and transfer calipers.

With the former, the jaws are closed against the object to be measured, and the reading is taken directly from the scale on the tool. These are commonly used by machinists for precise measurements down to one one-thousandth of an inch. My most frequent uses for these are when reloading and home gunsmithing.

Transfer calipers are similarly closed against an object, but are then compared either to another object, or a ruler of some sort. While less precise, these can be used to more easily measure awkward or irregularly shaped items. These are commonly used by wood workers, especially for lathe work.

Both types of calipers can be used to take inside, outside, and depth measurements. A single dial or digital caliper can usually perform all three tasks, while there are dedicated transfer calipers for each.

A selection of calipers and micrometers

Micrometers

A more limited yet more precise measuring tool than calipers, micrometers are also available in dial and digital versions. Capable of precision of up to one ten-thousandth of an inch, these are used for the most precise machining environments. Using and reading a traditional screw micrometer requires some practice. While I own several of these, I find a dial or digital caliper meets most of my needs.

Informal Measuring Tools

There are also everyday objects that can be used for rough measurements, such as an American dollar bill, which is nominally six inches wide by two and a half inches tall (more precisely, 6.14 inches wide by 2.61 inches tall), or American coins. A quarter is just under an inch in diameter, and 1/16th of an inch thick.

Many other common objects can be used to estimate dimensions, but a proper measuring device is always better. I carry a small tailor's tape measure in my jacket, and the multitool in my pants pocket has a scale on the sides.

For some history on measurements, I highly recommend the book Measuring America.