Rulers

Rulers come in various sizes. Often it is not even important whether a measured rule is used; usually all that is needed is a straight edge, which can be the edge of a ruler as easily as a piece of folded paper. Defined as the shortest distance between two points, there is no geometric proof for straightness. Rather, straight is an axiom, a given, that we often use to create a feeling of precision in drawing. Too often it is overused, but when you need a straight line, there is nothing like a ruler to help you make it.

There are many different commercially produced straight edge drawing utensils. These range in size from short "mini" rulers to longer yardsticks and beyond. With a right angle lip at the end, we call such a device a t-square. These are used when we need something drawn squarely with a drawing surface like a drafting table or a drawing board. Triangular based rules are produced as "drafting scales." These tools commonly have one edge marked in inches, and others marked to other scaling increments. Fancier models even attach to large swing arms or roll evenly across the drawing surface. Here are some different looking versions of the common "straight edge" drawing tool.

Arguments can be made for or against the metric system. I personally like and prefer the metric system in most cases, but we cannot always have our way. In some parts of the world the English system is standard. With this in mind, here are some common measurement conventions and tips.

When measuring less than an inch with a ruler that begins its first inch exactly on the end of the ruler, use the second inch instead. The distance from the 0 to the 1 is the same as from the 1 to the 2, and a chewed up or inaccurate end can make a big difference in a small measurement.
Inches are broken down by halves. The longest measuring lines are at the whole inch markings and are numbered. The second longest lines are the half inch markings. The third longest lines are the quarter inch markings (half of a half). The fourth longest lines are the eighth inch markings (half of a half of a half). The next smaller increment is a sixteenth, smaller still would be thirty-seconds and sixty-fourths (neither shown)

There are twelve inches in a foot, and three feet (thirty-six inches) in one yard.
If possible make all of your calculations in either the metric system or the English system. The less you have to convert one to the other, the less likely your calculations are to be inaccurate due to round off errors.

The metric system is based on the theoretical diameter of the earth. Once broken into segments, one meter represents one fraction of the spherically calculated distance from a polar cap to the equator. Meters are broken down into ten equal length segments called decimeters, decimeters are broken into ten segments called centimeters (one one hundredth of a meter), and centimeters are broken down into ten millimeters.

When creating precise lines that must be square with others, always make at least two measurements and connect the dots. Guesswork does have its place, but if your work must be precise it must be well measured.
If using a pen with a ruler, always try and use one with a metal furl that protects the edge of the ruler from ink leakage. Ball point pens and certain felt tips are built for this sort of work. There are pen rulers with slightly raised lips on the edge to minimize ink bleed as well. If you do not have such a ruler you can make one temporarily by placing a piece of masking tape on the back of a normal one. Since the tape will tend to collect dirt, this is not recommended for long term use.

Using the Proper Drawing Angle

When using a pencil with a straight edge, make sure that your point is sharp. Dull pencils can give wide and indeterminate lines. There are also special drafting pens that are designed to be used with straight edges. These pens will typically have a metal or plastic sheath that protects the tip from actually touching the edge of a ruler. Certain types of ruler make are easier to work with for different tasks, here we will deal with flat edged, chiseled edges and beveled or "lip" rulers.

When using a flat edged straight edge, keep pencils pointed into the corner for the straightest line, and keep pens directly upright for the least ink bleed.

Chiseled edged rulers are best for pencils only, and work best then the pencil is held sharply perpendicular to the edge.

Rulers with a lip are generally best for ink.

Using a Straight Edge With a Pen or Brush that Bleeds

To use a ruler with a dip pen or sable brush, place one finger under the forward rim of the straight edge.

By tilting the nib of the pen or furl of the brush across the raised edge of the ruler, a fairly straight line can be drawn or painted.

Note: This is not necessarily a precise method, but it works.