The ruler is normally a flat device with one or more straight edges, often made of plastic or wood. It comes in various shapes and sizes, the most common one being small, rectangular and ten to 20 centimetres in length. This length is marked on either or both of the straight edges. Other common shapes are triangular and semi-circular, used for measuring degrees.
Drawing straight lines - The most basic function of the ruler is providing a flat edge beside which it is easy to draw straight lines. Drawing geometric figures of this sort is not, however, a function solely occupied by the ruler. Any object with a flat edge can be used for this purpose. Using the ruler in this fashion can in turn be used for drawing tables, coordinate-axes and other objects in which straight lines are utilised.
Measuring things - Many rulers can be used for measurement. Here, however, it starts getting complicated. A standard ruler - or at least a good one - has two different measuring systems, one on each side of the ruler.
One side is occupied by the standard metric system, as imposed by Napoleon Bonaparte in an attempt to be different from the English. The number of centimetres varies from ruler to ruler and is usually, as mentioned above, between ten and 20. The length of the centimetres are another thing that vary from ruler to ruler. This is largely due to the fact that rulers are incredibly cheap to manufacture, and that the manufacturers in question are more interested in earning money than they are in correct measuring.
The other system is an English one. More ancient than the metric system, it consists of feet and inches. The inches, unlike centimetres, aren't always divided into tenths. They can also be divided into eights and sixteenths. The length of the inches vary in much the same way as described above.
Advanced and/or Annoying Uses
Medieval warfare - Being flexible tools, especially the plastic ones, rulers can also be employed as small catapults. In certain school classes, this is in fact, the primary usage of the ruler.
To use the ruler for this, you must first have a small object, such as a small piece of rubber. Then, after placing one end of the ruler in your left hand and placing the small object on the other, aim the ruler in the general direction of what you want to hit. Finally, if you pull the ruler towards yourself with your right hand until sufficient force is acquired, and release your right hand.
A slightly different way to catapult objects using a ruler is to take advantage of the laws of science and leverage. Place the ruler so that about half of its length or less is over the edge of your respective desk or table. Place an object on the end that is still on your surface. Hit the other end of the ruler as hard as you can to see the object - and the ruler - fly.
Swordplay - A slightly more infantile use is to use the rulers as miniature swords. This, however, is not recommended, as it includes the possibility of taking someone's eye out.
Gum scavenging - The flat rectangular shape of the ruler comes in handy if you feel like scraping gum from underneath tables an such. Though no-one is forced to follow advice given in any of the entries, it isn't really a very good idea to use the ruler for any of the basic functions following a gum-scraping session.
Making music - By placing the ruler halfway off of your desk or table as mentioned before, you can make odd and annoying noises by holding the end that is on the table and flicking the end that is off it.
Pain - Rulers can be quite a source of pain, as well. Before the age of 'political correctness,' rulers were often used by schoolteachers in hitting the knuckles of disobedient pupils.
In addition, rulers that have a metal edge on one side can be heated by rubbing against the sole of your shoe and then used to burn others.