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What are the parts of a rule?

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  A rule is a simple measuring device
A rule is a simple measuring device which can also be used to rule straight lines. Rules are used to measure distances and rule straight lines. Most standard rules resemble each other, but there are many rules with different designs and features.

For more information see:  What additional features can rules have? and What are the different types of rule?

Straight edges

The straight edges of rules can be used to draw straight lines or take measurements A standard rule has two long straight edges. These edges can be used to draw straight lines. They can also be used with a sharp instrument to cut or score straight lines.

Gradation marks

Rules have measurements marked in gradations on their edges The gradation marks show the units of measurement and their subdivisions. These are etched, engraved, drawn or moulded onto the rule and can be used to measure different lengths. The most common rules will show centimetres and millimetres on one edge and inches and their subdivision on the other.
Gears are used to mark some rules precisely A good rule should have gradation lines which are uniform widths, which makes accurate measuring easier. If the gradation marks have been machine etched the manufacturer will have used a gearing system to space the lines evenly and at the correct distances. The gears are precision set to move specific amounts as they turn and mark out accurate gradations.
Wonkee Donkee says "The process of using gears to machine etch gradation marks was developed around 125 years ago."
There are different marks on some rules, for different uses There are a number of variations of what gradations are available on different rules. This includes marks which show scaled measurements, and shrinkage allowances, which compensate for materials shrinking after they’ve been cast. A rule may also have other information scribed onto it, such as conversions or mathematical formulas.

For more information see: What are the different types of rule?

Hanging hole

One feature some rules has is the hanging hole Some rules will have hanging holes, so they can be hung up. These are used to store the rule safely, to prevent it getting damaged.